That number doesn't seem entirely accurate there are several game sizes across different regions at the moment.
The US eShop website says 25GB
The US eShop on console says 13.1GB with a notification saying total space required for all 3 games is 22GB
The US eShop has each game listed individually:
GTA III - 2.2GB, Vice City - 6.5GB, San Andreas - 10.8GB
Total - 19.5GB
The European eShop says 13.1GB
The European eShop has them individually:
GTA III - 2.2GB, Vice City - 119MB (likely wrong), San Andreas - 10.8GB
Total - 13.1GB
I think the most likely outcome is San Andreas + GTA III on cart at 13GB with Vice City's 100MB been there as an icon and downloads when you insert the card. Hopefully I'm wrong and it comes as an actual complete game on a 32GB cart as this game will hold a high price and make Rockstar a ton of money so no excuse to not use one really.