This is one of the reasons I’d have thought she’d want to keep a relatively ineffective Labour with decent numbers in opposition - if she has a large majority, she has to own everything and has nothing in her back pocket to keep the back benches in check, plus it would probably be enough to tip the balance against Corbyn, meaning the possibility of a more effective opposition leader - which would actually be more troublesome for her than a divided opposition is at the moment.
The only reason I can think for her going for an election is if she’s worried that she can’t keep enough of her own troops in line through Brexit negotiations and needs more numbers.