Interesting - I think I'll compare the dimensions.
I already did, and here they are. Differences are noted in parentheses.
Pontiac Vibe, base 2.4L Base Mazda3 Sport
Engine 2.4L I4, 158hp 2.0L I4, 148hp
Transmission 5MT 5MT
Wheelbase 2600mm (-40mm) 2640mm
Length 4336mm (-154mm) 4490mm
Width 1765mm (+10mm) 1755mm
Height 1549mm (+115mm) 1465mm
Weight 1350kg (+63kg) 1287kg
The Mazda's actually bigger in some dimensions than the Vibe, most notably 154mm more in length. The Pontiac has a slight power-to-weight edge at 8.5kg per hp as opposed to the Mazda's 8.69. These cars are very closely matched.