Gotta go with what someone posted earlier, you have to remember that your car is about 12 years old while your mates is 1 year old. Engineering has moved on quite a bit in that time. My brother was busy telling me how much quicker his 3 year old A3 TDI was than my 98 GTI - the very car he has had remapped so surprise surprise there. Best bit was the turbo blew up the other week

You also gotta think when you were "racing" your mate, he might have been thrashing the b******s of his car as well. If he was in front then he had a head start which isnt going to be easy to recover from.
I would have thought that the Golf is a good bit heavier that the corsa as well. My missus had a polo and that weighed an effing ton, nightmare to steer as it didnt have power steering. Power to weight ratio for both cars might then tip in the corsa's favour.
I have always found corsa's (and nova's) with 1.2 engines to be quite nippy up to about 50MPH then they start to lose it but can still do 70-90 before they struggle.
Personally dont have any complaints about my Mk3, only thing that has left me for dead was a scooby - so no surprise there.
If you want something nutty quick get a scooby or a bike. Brand new quick bikes that cost less than 9 grand will leave ferrari's (200K plus) standing so your 12 year old golf being beaten by a corsa aint that bad.