The editor? That would be because of the inclusion of two copies of the graphic sets (traditional, nearly-unmodified ones (apart from steel added to Bubble set, and a few extra objects and extra steel sizes to the Xmas set) for traditional Lemmix editing, and the NeoLemmix-ised ones). Since it included these in both the normal DAT format, as well as the Lemmix Editor compiled format (which is quite large), each new graphic set - especially if it was duplicated over both variants, which is the case for most sets, apart from the LPIII ones and the Doomsday Lemmings style - added a fair bit to the file size.
The latest release removes the need to compile graphic sets, thus cutting their large size out of the total filesize, and bringing it down.
