The tanks swell . the problem for me is it makes the tanks difficult to fit, and can do a little damage to the screen.
The fix , which has seen many miles and mixed terrain, is to loosen the whole headlight/ speedo support from the headstock. 2 bolts. This will then be nicely supported by the actual tanks themselves and leaves the headlight /speedo support bracket "floating" free from the main frame which in turn allows the tanks to do what they must.