Size doesn't really matter - it's more a question of how likely Amazon is to mirror broad demographics. If Game is seeing sales in close to same ratio as Amazon then the Amazon charts will be pretty accurate irrespective of volume of sales.i heard game.co.uk is too big to only use amazon as an indicator.
If Game and Amazon are used by widely different demographics then one may well be a poor indicator of the other (or trends in general).
I'd say the use of Amazon in US/UK has mostly worked out so long as the bundles, prices and demographics are broadly in line. However if you have say Game with a great bundle Amazon doesn't have then the similarity of their sales patterns will differ, possibly to a great extent.