Hello, general size limits is not any issue. Nothing to do with forum software or server storage\bandwidth, but an important choice & feature:
to show respect towards those who do not have access to fast internet connections & to maintain an overall snappy forum speed for all visitors and members. With images still looking great. The limit is a general\overall upload limit, thus including avatar uploads as well as normal attachments.
All info > "Why the 1,5 MB limit" & "Avatar" in
https://www.a7a10.net/forum/index.php?topic=427.0Same topic contains many recommendations for easy picture resize.
Good luck, you'll get your avatar in place in 2 minutes.