They probably just put extra weight (fuel) in the grasshopper for easier testing. The F9 on the other hand is basically an empty soda can with engines at the bottom and small fins at the top when it’s landing so even when firing just a single engine at minimum power it will generate too much thrust to hover.
I don't think it was exactly the same engine, and Grasshopper didn't have 9 engines, it had one. The engines are a surprisingly large mass fraction of a dry first stage.
https://www.google.com/search?q=spacex+grasshopper+video&num...