I did some crude comparative measurements of my colleague's MINI and my 500. While I cannot guarantee accuracy, they are close enough for discussion. I am quite surprised how close the measurements are between the two. I really though the MINI was significantly wider. (Excuse the lining up of the columns, it works in preparing the message but not the outcome).
MINI FIAT
Interior
Distance between:
Armrest 49.5 48.5
Upper door Sill 50.5 50
Glass Bottom 55.25 54.5
Glass top 43.5 47
Dash to hatch 77 79
Rear leg seat fwd 9.5 11
Rear leg seat rtn 0 3
Exterior
Hatch length botm 15.5 19
Rim to rim width 64 63
Body width 64 63
Height 55 59.5
Giallo Sport "Enzo" (formerly PE 311. We don't need no steenkeeng badges)