I have recently upgraded from PD10 to PD12 and notice a "Smart fit" option in Create disc options screen.
heres my question:

My original camcorder footage is 1080/50p and is 3590mb in size (this is the size of the original folder containing several avchd clips, before importing into PD)

I import into PD and put all clips onto time line, totally unedited

I click create disc

I notice that bluray and avchd options don't support 1080/50p
so i check alternative burn options and heres the results:

Baring in mind my original undeited footage is 1080/50p

Blu-ray disc
1080/50i reduces the original footage down to 2874mb
720/50p reduced to 2791mb
Smart fit is however incresed to 4233mb

AVCHD disc
1080/50i reduces the original to 2874bm (as with the blu-ray option)
720/50p reduces it to 2791 also same as with BD
Smart fit is however 3121mb, this is closer to the original 3590mb of the 1080/50p camcorder file

So whats going on here? how is smart fit creating bigger file sizes, and at what resolution/frame rate is smart fit burning the disc at?