Looks wise i find both of them definitely very different. Fabia looks a little more stretched. Besides I find swift a lot more sportier. my personal favorite.
but comparing the specs
SWIFT DIMENSIONS (swift website)
Overall Length 3,695mm Overall Width 1,690mm Overall Height 1,530mm Wheelbase 2,390mm Track Front
Rear 1,470mm
1,480mm Ground Clearance 170mm Minimum Turning radius 4.7 metres Luggage Capacity 232 litres (with seatback raised)
368 litres (with seatback folded)
Fabia Dimensions (l/w/h) 4232/1646/1449mm
As the body size comparison clearly shows swift is shorter and less wider but taller than fabia. The basic skeleton may have similarities but the final look differs.
For more detailed comparison check
The new Skoda Fabia - Feel Special