Flip her onto her side and hold one of her legs upward. You’ll get compliments because it’s a bit athletic and does in fact feel deeper.
Important: try this only if you know she’s into being handled this way. She likely is if she’s saying things like that, but always make sure your partner enjoys stuff before adding it. Also, the usual cautions re: PIA vs PIV apply.
Details below. (Edit: CWs added)