If you can not share the file then look at the before-after properties.
If it is an animation, the number of layers (frames)
Does the new gif have transparency.
If the old file is optimised and you un-optimised for editing Maybe re-optimising not as efficient.
The one thing that can get larger is the color map. How many colors does the original have compared with the new file. Open the colormap dock Windows -> Dockable Dialogs -> Colormap
If it is an animation, the number of layers (frames)
Does the new gif have transparency.
If the old file is optimised and you un-optimised for editing Maybe re-optimising not as efficient.
The one thing that can get larger is the color map. How many colors does the original have compared with the new file. Open the colormap dock Windows -> Dockable Dialogs -> Colormap