Beam's don't get stronger with PL alone, they get stronger as your PL begins to exceed the server average PL. In other words: If you have 10,000,000 PL, and everyone else on the server has 1,000,000, you're generic beam is going to be more powerful than a 1,000,000 PL kamehameha, and your kamehameha is going to take out most of the map.
And I should know. Several of the advanced players who use Buu once in a while should have experienced this before. On one map in one server I played (with a max players of 20) the server average consistently stayed between 1,600,000 and 1,800,000 PL. I was playing Buu, and using candy attack on about 3/4ths of my kills to gain PL. After about oh... 2 hours (I had nothing to do) Buu was at about 10,000,000 PL. Then I transformed. Then I gained more PL. I ended up capping out at around 18,000,000 when the saiyans on the server had just started transforming (lots of people + me killing you every 5 seconds = low server PL average), and believe me, when you have 7 or 8 times the server's average PL, and you're Buu of all characters, you've got some advantages. My generic beam detonated larger than a kamehameha, which frustrated several players, and his mouth beam was so powerful that it actually detonated killing everyone on more than half of the map (although it was a smaller map, that space one... can't think of it's name), and it was even so powerful that it killed everyone in that building unconditionally, even through the floor and walls.
So to answer your question again: Yes, PL affects beam size and power, but not by shear numbers. Only be comparing yourself to the server average.