Just bought myself my first full face. I normally wear my XC (Hex) helmet quite loose on the strap and not to tight on the clasp/ratchet. I've tried a few full face ones in the past and this feels nice but its not too tight. It doesn't move from side to side but does roll back a forward quite a bit. I measured my head (58cm) and bought the 56-58 size.
Should my head feel VERY cosy/crushing my cheeks tight in the helmet or am I overly concerned?

should push cheeks in a bit

does your helmet come with extra pads so you can add some to the front. everyones head is different. mine's is a fat head so i usually have to add padding to the front lobe area.

you do need it quite snug. shouldn't rock back and forth so much.

Do the strap up . tip your head forward and try to pull the helmet off from the back by rolling it forward? If you can pull it off its far too big, if the front of the helmet hits your nose then its too big. Try the same backwards. If it digs into he back of your neck its too big.

Without the strap done up if you shake your head it should not move on your head more than a tiny amount and as jedi says it should squeeze your cheeks a bit

Try the same with your Xc helmet as well – sounds to me like you wear that far too loose – like most MTBers I should add.

Fir is all with helmets – one that is too loose is virtually useless and could be activly harmful

IMO / IME most mtbers wear their helmets far to loose.

I challenge you all to try the pulling the helmet of forwards – most will fail that.

Hold the mouth part and move left and right
if it moves then to big.
To tight will be a struggle to put it on
or clamping feeling around the cheek bones

A snug fit is what you are looking for.

Tried all TJ's suggestions. Cant pull it off the front even with the strap undone. I would say its a good comfortable fit.
Thanks for the prompt replies.