My speculation is that Google intentionally keeps the size of their Product Listing Ads small enough that no real evaluation can be done from the search results pages. Thus the searcher needs to click through each and every listing that piques their interest. They do this because Google charges per click, not per impression. If Google charged per impression you can be sure advertisers would be up in arms about the size of the images.
Let's evaluate:
None of these images gives me enough information to make an informed decision. They give just enough to make me want to get a better look. It is very likely that Google gets multiple clicks back and forth (or for the more tech savvy searchers they get multiple tabs opened in rapid succession) for the majority of queries where a searcher clicks on a Product Listing Ad.
Facebook, on the other hand, charges for impressions. Some might argue that as Facebook advertisers we bid on conversions or clicks or engagement or add to carts, let me stop you right there. No matter how Facebook tries to optimize your spend, in the end they are charging you for impressions.
Here are a few of my ads for Facebook (For the sake of a more apples to apples comparison I'm only going to evaluate Facebook's Dynamic Product Ads, which is the closest thing to Google's Product Listing Ads that Facebook offers.):
What you see above is the right hand bar of Facebook. It's practically a throwaway ad unit for them. And yet the pictures are much bigger than what Google shows for their main attraction.
Above you see a full size Dynamic Product Ad. With the right cropping I can get my images to the point where anyone can evaluate their interest in a product without having to click through. This is vital for me because the user is not searching for my products. I'm targeting them based on various parameters that Facebook allows me to set.
Because Facebook charges you for impressions they have motivation to keep users on the site or in the app. They also have motivation to make each impression as valuable as possible. Think of the difference here. Facebook is motivated to increase the value for both users and advertisers. Google, on the other hand, has only their own motivation in mind. The experience they present is optimized for Google at the expense of both the searcher and the advertiser.
I am not long on Google, although I do believe they likely still have some growth ahead. They have one product that makes money, and quite frankly it looks like garbage. They have as much or more data about people than Facebook has (that will happen when they own the dominant smartphone operating system, dominant browser, dominate email service, dominant search engine, dominant map software, etc.), but they have three things working against them. And I'm pretty much convinced that these are not fixable flaws unless there is an extreme cultural change via removing the people in charge.
Flaw 1: Terrible Ad Units
Facebook ads can be shared, commented on, and liked. Each of these actions increases the value of the ad for the advertiser and the Facebook user. I've had hit products because I read the Facebook comments and acted on customer suggestions. I've learned why some products were popular when I just thought they were a cool picture on a t-shirt. Knowing why allowed me to replicate that success rather than have a one off hit. Every Google Ad lives or dies with the one user who saw the ad. This is huge.
Flaw 2: Overly complex Advertising Tools.
The majority of my peers outsource their Adwords accounts to agencies because it's too complex and too boring to manage themselves. Agencies, as a rule, suck. Most agencies subscribe to the same software packages to manage accounts, and chances are the person working on your account has been on the job for 3 to 6 months. No one is working hard to improve the quality of the ads. My peers obsess over how to make better Facebook ads, and success is easy to see since I'm bidding on impressions and can get impressions near immediately. I don't have to wait for someone to use the search query I'm bidding on.
Flaw 3: Search will be less important as time goes on.
As algorithms continue to get better the tools we use will know what we need at the right time or maybe even before we know we need it. Google is like a librarian. They will help you find a book you are looking for or even one you aren't looking for in the case that you know a genre to search for. Facebook allows the authors to target you directly. When you buy the gardening book from the author as a result of your Facebook post you won't need to search for a book.
In summary, as a e-retailer I really dislike Google. I love Facebook. As an investor I don't own Google/Alphabet directly. I know my retirement account mutual funds own some. I believe that Google has long term issues and unless they find a way to better monetize their vast trove of customer data via their vast array of dominant software offerings they will start to decline in revenue. My belief is they've already started to decline in quality and innovation but they've made up for it by squeezing more dollars out of each search intent per customer (don't want to say query since I'm not sure if Google would count a user going back and forth between a SERP and websites one query or many).
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Update 1/23/2020
I still feel the same way about Google as I always have, however I have purchased around $10,000 worth of shares this week. The reason I made this purchase is because they will be able to continue to extract rents on the overall internet for a long time. I’d love to see them broken up so that one company doesn’t control so much of the software that runs the internet (browser, search engine, android, gmail) but even if all of those things were separated I believe each individual product would improve if they weren’t all beholden to what is good for Google as a whole.