I have seen numerous people wearing helmets so loose they are like rattle cans.
The helmet should be snug on your head - not compressing it but snug - and it should contact your cheeks firmly, It should be strong and heavy material, its supposed to save you - not get you out of challans
Ive seen helmets so loose peoples faces are like 2 inches away from their helmets, That is like asking for trouble the equivalent of getting punched in the face.
A little insurance I like to do is get a permanent sticker made with your name and bloodgroup and any allergies stuck on the helmet outside - and lacquer it onto the thing. Just pray you never need it.. When you are on the road unconscious - that little sticker can save your life..
I still have my broken Indian STUDDS helmet - with the sticker on it in Pakistan.
And for the love of God do not leave your helmet lying on the ground or something filthy - I always used my Helmet lock on my bikes. they are supposed to keep your helmet in place.
For the techies - put a mic and speaker in the helmet for a great handsfree phone system