You're using siege engines, right? If you have enough siege engines, you only really need 1 actual attacking unit (axemen/swordsmen/macemen/rifleman/whatever) for each defender - though more is prudent.
"For small maps" isn't really much to go on for giving you advice - Gandhi will be a pushover on whatever map type, and you can always expect to have to chew through plenty of units when attacking Ragnar, or have a nasty time taking one of Saladin's cities on a hill. Basically, look around with a spy before going to war, and try and have more attacking units in your stack than the enemy has in its best cities. If they have lots of horse units, bring several spearmen/pikemen to defend your stack. Bring a few longbowmen (if it's the right era) to defend cities you keep. If you're fighting longbowmen, make sure you have at least one macemen for each longbowmen, and a good number of siege engines.
The biggest thing is not just having a huge stack - if you spend all the time building up a stack big enough to wipe out the entire enemy civ, it'll cost you a fortune to upkeep before you even go to war, and be all the closer to being obsolete by the time the war starts. Check the first few cities, build up enough basic attack units to take out just those cities, bring along defenders (spearmen/archers), plenty of siege engines, and keep replacing things as you go. That's my basic formula for a successful war - you'll learn many of the other tricks as you go along.
