Is it ok to use Frontline for dogs on cats?

My last vet said it was fine and told me to use .5mL for the dosage. I never ended up needing to then, but now I think I do. However, my current vet says this is not recommended (info came as a call-back, so I didn’t have the chance to ask follow-up questions).

I checked and the main active ingredient, fipronil, is in both at the same concentration. And the second active ingredient is slightly less concentrated in the dog solution than in the cat solution (8.8% vs 11.8%). So why would the dog solution not be ok for cats?