Kangmin7 is right.
I was looking into this before. I wanted to make a Scrafty with Drain Punch and wanted to see what Big Root would do.
Drain Punch deals damage and takes 50% of damage dealt to heal yourself (as HP).
The other 50% of the damage does nothing. What Big Root does is take an extra 30% out of that 50% and adds it to what you get back.
Drain Punch is still 75 BP, so no, Drain Punch does not become more powerful.