They might have been asking about methane production, that's a big topic for cattle because it's a very potent greenhouse gas and having a lot of cattle produces a lot of methane - and it's a byproduct of how their digestive systems work and all ruminants, sheep included, have that problem to some degree.

I wasn't able to find a definitive answer, but per animal sheep aren't as bad, but it's due to their smaller size and if western countries kept eating the same amount of meat but ate more sheep (due to it being cheaper from the demand for sheep condoms, lol) instead of beef I'm not sure it would be better. (Replacing chicken with sheep would definitely be worse though.)