Best way to reduce a GIFs file size without losing much quality ?

I just recently started messing with GIFs for some marketing work of mine and I''m noticing that it's really hard to shrink the file size much for them with all the website tools I have used. And sometimes I upload a large GIF to some of those sites to reduce the size and it can't reduce it any ? Also whenever I try to change the aspect ratio of GIFs I notice that it makes the file size INSANE even if the resolution is actually smaller overall than the original to start with. Any help guys ?