Her top is a bit too small

"Hey guys thanks again for letting me move in with you two" Frisk said, trying to start some casual conversation while they began unpacking her belongings in the spare room.

"NO PROBLEM HUMAN, I, THE GREAT PAPYRUS, AM HONORED TO HAVE YOU LIVING WITH US" Papyrus boomed in response.

"yeah it's no big deal, we're happy to have you" Sans responded

Frisk smiled at the kind replies the two had given her. Her unrestrained breasts bounced up and down as she sashayed over to one of the boxes in the corner of the room. However she really should've lifted with her legs rather than her back, as when she lifted her upper body back up, her shirt was resting above her nipples, and everything from the top of her small pink areolas to the soft curve of her perky C-cups was on display.

Being preoccupied with unboxing, neither of the skelebros noticed this, even as Frisk walked to the doorframe and called back to them.

"How about we take a break and go out for a bit huh?" she said in between trying to catch her breath from the heavy lifting.

Had she not had her body facing away from the two of them, they may have had caught her before they went out anywheres. Unfortunately, this wasn't the case.

"Sure" both brothers said in unison, as they grabbed their things and followed behind.