This is mainly so that emotes will not be so disruptive to users on other instances. Due to how they are implemented, most of the emotes have the effect of flooding the comment sections when viewed from other instances, and due to the large amount of cross-instance posting, this is a real issue that makes even sympathetic users annoyed.
Downsizing can be done pretty effectively with an automatic script, using something standard like ImageMagick to downsize them. So, it should not be hard or timetaking for the devs to do.
This will also decrease their filesize, making them load much faster for everyone!
theres a lemmy fix incoming in the next 0.19 update which will allow frontends to size custom emojis consistently
That's good to hear, I haven't kept up with development in the last few months.