We probably all have a pretty good idea of the answer to this question. For example, I know for sure I do, and I'm pretty sure Stoat does.
And I'm pretty sure that my answer and Stoat's answer are close enough that we can talk about Big Science without immediatly misunderstanding each other. I am sure, however, that my answer and his are not identical.
Each of the rest of us are also going to have answers that are not identical. We can either ignore these (not necessarily) small differences in the way we think about Big Science, or we can try to come up with a common definition. (And then try to always use it.)
We do not necessarily have to do this before we begin discussing things. But we ought to keep in mind that this problem exists, and work on it from time to time. Especially when we perceive a problem elsewhere that might be the result of conflicting ideas about what Big Science is.
Thoughts about this are solicited.