The pulling of the springs at the back has to be balenced by the pulling of the strings at the front....if you want you trem to float (balence in the middle). If its a strat type trem you can put extra springs on and the trem will sit flat on top of the body and will will help to keep the guitar stay in tune really wel. lIt will feel much stiffer to push down and can be adjusted to not able to push down at all.. If its a locking trem (floyd rose) putting extra springs on will cause problems unless your knowledgable about setting up guitars....you could also think about a trem stopper if you want your locking trem to not balence in the middle. Heavier strings will make it stiffer but again you will have to set the guitar up to suit the new gage strings.
I hope this helps.