If you have 50 HP staked and you stake 50 HIVE, you are getting double the VESTS. Because the voting power depends on the used VESTS, you get 50% in this case.

If you powered up 100 HIVE (for the 50HP case and 0% upvote power), you would have moved to 66% Upvote power, and so on...

Imagine like this, if you have X HP, the % of voting power is relative to that HP... so if you add more HP, that percentage changes accordingly.