"… As suggested by the title of an early manual Inside #MacPaint: Sailing through the Sea of FatBits on a Single-Pixel Raft, FatBits was a killer feature for MacPaint. For Susan Kare, Apple’s in-house graphic designer, the mode accelerated the development of Macintosh’s small bitmap icons where every pixel mattered.
"Marketing pushed to rename “FatBits” “Magnify”, but Atkinson won the argument, arguing that the name gave the program some personality (Young 1985).
"FatBits is restricted to a single zoom level and, within the code, is handled via many special-cased changes to the input and output. The single zoom level restriction was due to the limited hardware; an arbitrary scaling factor would have required too much CPU…"
https://ztoz.blog/posts/macpaint-source-code/