My understanding is that it depends!
Are you field length limited?
If so, if you are ASDA limited then V2 decreases with increasing weight. This is because you need more distance to stop.
If you are TORA/TODA limited then V2 increased as weight increases. This is because you need more time on 2 engines before one 'pops'.
If you are not FLL, then your V2 can stay the same as you increase weight until you do become FLL. Then refer to the stuff I said above.
I think.