Clicking off and back onto the same avatar without changing the upload can sporadically change the size, which seems like a client side issue.
This applies to both personally uploaded avatars, as well as avatars listed onto the avatar marketplace.
I've confirmed this issue does not just apply to me, as attatched is a picture someone sent of one of the listings with the avatar preview shrunk down really small, and then on my end when checking immediately after, it's fine and full size (also attached). Shortly after I let them know on my end that I couldn't reproduce it, it resolved itself on their end when they re-selected the avatar.
These pictures are from the previous build, but the issue still persists on the current build.
I eventually ran into the issue again when I was continuously iterating uploads to resolve other avatar preview issues mentioned in this canny (https://feedback.vrchat.com/avatar-30/p/vrc-update-breaks-avatar-menu-preview-sizes).
It occurs whether I change into the new avatar upload or not, whether I switch avatars via the wing or not, or whether I change selections from the larger Avatar menu without switching avatars. I temporarily resolve it by restarting the VRChat client entirely, but eventually if I click off and back enough times or after enough time, it'll re-occur.
Another friend of mine clicked on the listing as I even write this canny, and reported the avatar preview is the smallest it's ever been. After restarting the client, it returns to its larger size.