Filesize for AVCHD has to do with storage space during capturing, not during editing.
It’s not the best to edit with because of it’s long GOPs. Correct me if I’m wrong, but isn’t it roughly 15 reference frames for every REAL image (I-frame)? so essentially, you only get one real frame of footage every half second. These reference frames take rendering, whereas ProRes uses real images for every single frame. Hence the large file size and the no-hassle editing in Final Cut.
Some may see this as an advantage, while others see it as a disadvantage. Depends on your situation.
Good Luck,
Eric Allen Films