Well the two kinds of resolutions I'm speaking of are size and the imagery placed in the size, its much like a computer monitor resolutions, for example you can have a 17" wide monitor and change your screen resolution from 1024 x 768 over to 1280 x 1024, same size monitor but the resolution has been changed, and in effect all the pictures with the 1280 x 1024 will appear smaller because more pixels (resolutions) are being added, the higher the resolution the better the quality but it makes the images smaller. So what I was saying is yes all your 8 frames are the same size (think of those as your monitor size) but the images on each frame were different thus making it appear like the frames are different sizes when all it really could be is the resolutions inside the frames.
I didn't find the resolution settings by any program, what I did was used a program to extract all 8 frames with my image program call Irfanview
from the animation gif and looked at them one at a time and then noticed by looking at each frame some were different resolutions. It could simply be the placement of imagery on the 256x256 frames that is different and its not a resolution issue, and yes the butterfly's being added could have altered each frame resolution regardless that the frames are all the same size because each butterfly is a different image and color.
The programs I use for animations I lost when I reinstalled windows, so now I'm using some online editors until I get some new ones, but quite frankly these online animation editors are pretty damn good if you can find ones that doesn't leave watermarks, and there are plenty that do and don't. Right now for the actual animations I sometimes use this site online gif maker
and there main editor online image editor
and a few other, but the making of the frames I use a program on my computer called paint.net
not to be confused with windows paint, thats a free program and its a damn good one too btw, although it does not do animations, but you can makes your frames with it and do just about anything else but it steers clear of making the animations, but that can be done with other programs or places, its the making of the frames thats the hardest part. Those are links above btw just click those to check them out.