We mistakenly bought the wrong kind of dog food and already opened the bag! Our dog weighs 20 lbs. and the food we bought is for large breeds. Is it ok to feed it to our dog as long as we scale back the portions?

Should be okay. Just keep an eye on your dog in case he suddenly starts growing..... ;-)

for the short term it probably wont matter, as long as it was not a great big bag that is going to last forever. if it is only a pup be a little wary, because there could be a varient in the requried vitamins, minerals and more importatly protein levels. As far as feeding less, it is not the volume that makes the difference it is the diffrence in the formulation and the added or subtracted nutrients.

Hello,
I have to agree with nattie1976, go by what she said.
---Tiana