Yes, the AI has a serious problem with Huge units sizes because it simply builds and builds until its settlement populations are severely depleted.

The whole issue with unit sizes could be avoided if CA scaled the game's city/population system to match the unit size selected. Basically if you play with Huge sized units then starting populations for all settlements should be four times larger as well. To offset any problems with massive populations and growth the population threshholds needed to enlarge a given settlement would increase fourfold as well (i.e. with Huge sized units a Huge city can only be achieved by reaching 100,000 inhabitants instead of 24,000).