Castelli sizing...

It seems that vary from product to product IME.

I usually take a Medium.  My Alpha is a Large, and fits great.  As is my Gabba.  A Perfetto Light Short Sleeve in Large was too small; the Extra Large was fine.  But the Large Perfetto Long Sleeve is too large - I’m a Medium for that one!  Bonkers.

Castelli is Italian, Italians are generally a bit more petit than most Brits!

I am normally a medium in most things, but I wear Castelli in large!

None of Castelli’s upper body gear fits me. At 6’1” / 1.85m and 80kg, XL seems about right for length, but it’s just too narrow at the tops. I’m a bit carrot-shaped and the jerseys pinch around the armpits, whereas the jackets don’t have the range of movement to let me hunch over on the drops.

A pity because I really like the feel of their stuff, it looks good and is often decently priced (during the sales.....).

Evans, and I guess others have size guides. According to this I am a Castelli L whereas all my other clothing is S or M.

I'm a large in most things (6'3, 78kg) but in Castelli I often find myself umming and arring between XL and XXL. Usually order both sizes and send one back.

Nah. Perfectly normal if you don’t want every contour of your body on show. No one needs to see the outline of my nipples in my normal size. Also find the smaller sizes size up short in the body. 

I wear the same size in Castelli as I do in everything else (small), Castelli fits me better than most my other gear - it is more form-fitting than other brands (dhb for example).