If you are in a climate that has a "winter maintenance season", try taking the tanks off, emptying them completely (as in, bone dry), and let them sit as long as you can. The tanks should shrink back down some. It might also help to add some type of fuel additive that absorbs water on a regular basis - like stuff meant for boats. If it's like every other plastic tank, it's likely the water collected in the ethanol that gets fused into the tank material and causes it to swell. If allowed to dry completely, it shrinks back down. I would imagine that different plastic compounds are affected to a different degree also. If you have non-ethanol available to you, use it, and those of us who don't have it can be jealous.
"You do your own thing in your own time, you should be proud."
2004 KTM 950 ADV : : : : : : : 1998 Triumph Tiger
1999 KTM 640 ADV : : : : : : : 1993 Kawasaki KLX650
2007 Kawasaki KLX350S : : : : 1996 Yamaha XT350