if it worked like that i wouldn't be surprised given quirks can be super specific on their rules, i mean like her clothes grow and shrink with her so as long as the same rule doesn't apply to mt lady as rule (yes that's a hero student name) then she can shrink living things all she wants.

Base +18

You do realize... That Izuku even at that size, could kill her. Infact since the area would be smaller, and the power that OFA provides, he would be like a bullet that's moving on its own. Hell, he can lift 700 kg without OFA (he was standing on a Japanese kei truck the day of the entrance exam, on top of the pile of trash) and that's just at the start of the series. His punches, even quirkless, would generate more than enough force to Pierce through her skin and stomach tissue.

Tl Dr, if Mt lady actually tried this, Izuku would probably kill her, totally by accident, via internal bleeding/ blood loss.

Base +5