7 .
You are out for the day with a group of English friends, who have also brought along some other people that you don't know very well. In casual conversation you ask one of your own friends about one of these other people, and he tells you ~ almost offhandedly ~ 'Oh, Steve? He's all right really, but sometimes I wonder whether he isn't a few sandwiches short of a picnic.'
What is this supposed to mean?
Steve very rarely contributes as generously as everyone else when eating together; i.e., he's mean ('stingy', even).
Steve is absent-minded, didn't make or bring along his own packed lunch, and is sure to be 'on the scrounge' for other people's leftovers when you all stop to eat.
Steve is not very intelligent.
Steve is a trainee caterer who has not quite finished his course yet.
'A bull in a china shop' conjures up the image of 'an accident waiting to happen' (as we also say; think about that!).
Answers 3 and 4 also suggest unsuitable and/or futile things, but each of these is more usually used to describe things that drop or sink, including metaphorical uses such as when a supposedly persuasive performance fails to convince anyone (e.g. a speech or drama segment 'goes down badly'), or where a number of singers without instrumental accompaniment begin to slip off-key and 'lose pitch' (usually, downwards).