Sadly, what she doesn't know is that The Joker liked her bigger body for another reason. When the cops would come to arrest the Joker for his recent scheme, he would run like hell egging Harley to run as well. Harley would run about 6 or 7 steps and then she'd fall down exhausted, sweaty and out of breath.
This would then distract the cops, therefore giving the Joker extra time to escape and...I don't know take over an orphanage with Jokerfied Dogs or something.
Thanks, I was going for something more cutesy here, as opposed to their usual nutty relationship I've been meaning to draw Harley in her Arkham Asylum outfit, so hopefully you'll see that from me eventually! And I think you'd do an awesome cosplay of that outfit!