For obvious reasons you didn't tell her exactly that, only that you need a favor and your willing to pay a big tip for it. You took out a large wad of cash like some fishbait dangling over her.

She in return, smiled delightfuly, agreeing to it as she took the cash. "Wow! Never been tipped this big before, must be a pretty big favor if you tipped me that big. So, what do you need, dear customer~"

You ask her to follow you somewhere private and out of sight from the public, the casino has a bathroom for the staff, even if it's a girl's only one, you only needed a few minutes to shrink and actually be 

However a question comes to mind on whether you tell her about the shrink ray or lie...