It can be either, because people use both definitions. That's how language works (not trying to be sarcastic).

One big mistake people make around vocabulary/grammar etc. is that there is a 'correct' way of defining something.

A dictionary is not there to tell you the rules. It only describes how a word or phrase is actually used. That's why the Oxford English Dictionary is updated quarterly. If it wasn't that way, language would never change.

I personally always thought of a 'dirty' bulk as poor quality food (which naturally tends to overshoot calorie allowances), but I've seen it described as a huge surplus even if the food is good. It really doesn't matter which is which as long as people involved in the conversation have the same understanding.