I have a friend that to look at is of a very good build- Problem is that he is 6'6" tall. Nothing fits him. If it fits the Chest comfortably- it won't reach his waist and the Long sleeve shirts are up to his elbows. For his birthday- I bought him a Short sleeve top in XXXXL size- Fits the chest and as it is stretchy- it fits his waist (Which is expanding a bit with age) the length reaches the top of his shorts and he is pleased with it. In fact I quite liked the style so I went back and bought one in medium size- It does fit me but I will not be wearing it again. It does not wick moisture away and on last weeks ride- I felt cold.
Its not only size that varies on tops- the material will vary aswell.