Just the taste something sweet spikes insulin levels[1], and as you said at least some artificial sweeteners don't cause fullness[2] (I am amused that is not the major finding there, heh).

It makes sense, the body releases insulin in expectation of nutrition coming in, but no nutrition comes in, so the end result is feeling hungrier than before.

[1]https://www.ncbi.nlm.nih.gov/pubmed/17510492 [2]https://www.ncbi.nlm.nih.gov/pubmed/21255472