Of 145 .plan files currently in my reserve, the largest is 46mb and the smallest is around 5.5mb.
because of meta data, it isn't necessarily how much detail is in the plan or how many items per se', as it is how cleanly was it drawn.
If you have revised the same plan 15 times, even if nothing was added (just moving cabinets and walls) all that data is still saved into your file, therefore making it a much larger file than your first revision.
It is my impression that behind the scenes, the software operates as such: (not exactly, but you get the idea)
draw exterior wall
wall is 120" in length
wall is 96" high
wall materials are XXXXX XXXXX, XXXXXXX, XXXXXX, XXXXXXX
delete wall
note wall was deleted
undo
redraw wall
draw exterior wall
wall is 86" in length
wall is 96" high
wall materials are XXXXX XXXXX, XXXXXXX, XXXXXX, XXXXXXX
because we have the "UNDO" function, the software essentially will note every single click you make, BUT Undoing or Deleting doesn't make information disappear (if anything it will add MORE information) it is still embedded in the file, therefore making it larger.
OR AT LEAST ... that's my take on how the software works.