Most likely what happened is some MBA ran a short A/B test of smaller vs. bigger video thumbnails, and the A/B results showed more "engagement" with the larger size thumbs, and so, of course, to meet his/her performance goals, the MBA had the page altered to the version that showed "more engagement".
I think it also helps them figure out which videos keep people on YouTube longer. If I scroll to a section of the page that has 6 videos, and I stare at them for 10 seconds, then scroll down, they'll know that one or two of those videos must have been somewhat interesting. But if I stare at 6 videos, then scroll away 2 seconds later, it knows that nothing in that batch was worthwhile.
The fewer videos they have in focus at a time, the more accurate their algorithms can be.
