And for good reason. A responsible supervisor will typically give PhD students tasks that he has already "kind-of / sort-of" worked out as feasible. I once read the following recommendation for how to set dissertation subjects, I can't recall by whom.

- BSc thesis: what I can do over lunch

- Master thesis: what I could do in an afternoon

- PhD thesis: what I could do in a week

(Here the figures don't mean typing, programming etc, but the core intellectual work). After having supervised nearly 200 theses are BSc, Masters and PhD level, I must agree, this is a pretty great heuristic.

Note: there are exceptions. Some PhD theses go way beyond this, but those are rare. I have no idea about Yuan Cao's work, discussed here.