biscuit joiner

[andy100]

Can anyone advise me please,

I have recently bought a biscuit joiner from screwfix.
When I cut a recess for the biscuit and fit the biscuit in the recess it appears to be a very loose fit ie the recess is at least 1 or 2 mm bigger.
I have been given conflicting opinions on weather this is correct or not.
I understand the biscuits swell when in contact with the clue.

 

[masona]

I understand the biscuits swell when in contact with the clue.

That's correct and also to let the air out,best to experiment on a sample wood.