We're happy to have feedback or suggestions that will improve the site, the overall experience for all the users.
Having bigger/heavier avatars or sigs doesn't improve it in any way, on the contrary, it affects site performance. All pages will be heavier and load slower. Our servers would always have a greater load, leading to further performance hits.
If all this can be avoided by simply restricting the size of avatars, I'm sure it makes sense in all respects.
If you look back, a lot of suggestions have been incorporated into the site, and we have come a long way from where we were 10 years back.
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And of course, from an aesthetic point of view, the beauty of a simple still image cannot be compared to a flashy animation. I certainly cannot look at a page with all images constantly changing.