I agree with mac on the suggestions for coolant and additives
and I also agree, making the tank bigger won't do anything, as that isn't being circulated at all, its there to adjust for pressure changes and the likes more than to add any more coolant to the mix!
NOW< if you wanted to some how add a larger oil cooler? then that would sure help to make it run cooler
but since so many folks have ranchers and run all sorts of riding types and places and styles
and so few need them
I think you have other issue's going on!
if your running this atv at MAX RPM's all the time and think this is whats making it run so HOT
my advice, would be BUY a bigger atv that can handle things better, rather than trying to mod this one to meet your needs
as its only a small HP atv, under 30 hp, and the longer you run it at MAX, the faster your killing it?
WHICH brings me to this, have you done a compression test ??
once you start to get low on compression, you can have running issue;'s when things get HOT, things cool down, they can run better?
and just cause it did it since??? doesn't mean it was EVER good on compression, could have been low to start with and well??