I haven't played on huge but there has been a thread about it some time ago. It seems that apart from strategical changes (longer training time and more costly) there are also some tactical changes.
The unit sizes change between different numbers. For example: a unit of 60 on default will be 80 on normal and 40 on small. These fixed numbers aren't completly related to each other. So on small units there will be more cavelry in one unit in comparison to a unit of spears than there would be on normal.

So what does this mean for the huge size setting? It means that big numbered units will become stronger by comparison to smaller numbered units. In other words, infantry and especially spearunits, byz inf and the likes will become more important and stronger than they would be in a game on normal.