I'll try and help....my HRM is a bog-standard one, has a watch that goes on my wrist and a plastic bit containing the monitor that's on an adjustable elastic strap to go around my chest. It can be easily adjusted to fit me or my husband (who's considerable larger than me) so I'm sure you won't have any problems! You've probably just seen the monitor bit without the elastic strap attached.
I wouldn't be without mine now - even if it's just so I can have a good reason to go even slower than usual!