Why can cutting sugar make a cake dense instead of simply less sweet?

Answer: Starch sets too early

Starch sets too early ✓ — Right: with less sugar, starch and proteins can set too early. In a normal cake, dissolved sugar ties up water and delays that setting, giving bubbles time to expand before the crumb locks. Cut too much sugar and the cake may firm before it reaches full volume, eating dense rather than just less sweet. The trick is that a sweetener also acts like a batter clock.

Builds stronger gluten — Sugar usually does the opposite of strengthening gluten. By competing for water and interfering with protein interactions, it tenderizes the crumb; too much can even make structure too weak. A lean bread wants strong gluten, but a cake wants just enough support plus softness. Removing sugar can make the structure set sooner, not magically stronger.

Creates all the lift — Sugar helps create lift during creaming because crystals cut tiny air pockets into fat, but it does not create all the rise. Steam, carbon dioxide, and expanding trapped air also do major work in the oven. The dense-cake surprise is not just fewer starter bubbles; it is that starch and proteins may set before the bubbles finish expanding.