Basically yes - sizes in the UK come up pretty small. I found that out when I went to the States and tried on clothes which I thought were the same size as in the UK - how wrong I was.
The rule is - for tops if you're used to wearing an XL in the USA, then you should be buying a XXL in the UK.
The same goes for trousers/jeans - if you wear a 36in waist jeans in the USA, then buy a 38in in the UK.