RE: How mutch bigger sail?
No, this is quite normal.
Heavyweight sailors need more power and the best way to get more power i[img][/img]s to use a larger rig.
Also, the GO 150 is significantly wider than the other sailors 100-115 liter boards and the extra width (which helps you to get planing) requires a little more power as well.
Nice thing about this is that with the wider footstrap offset on your GO 150 it makes it much easier to handle a larger sail as you have better leverage (footstrap offset) to control the board and fin.
In Formula racing, you may see both heavier and lightweight sailors on similar size sails, but that's to take them upwind and downwind at the extreme angles that formula courses demand. EVen at the top levels, there is a trend for even the heavier guys to use a bit smaller (but more powerfully rigged) sails.
Hope this helps,