I think the OP has a point. The only times the AI has attacked with odds that are unfavorable to me have been when I've left settlements only lightly garrisoned -- a problem easily remedied by expanding slower and leaving strong border garrisons behind.

The hardest part of M2TW is managing the economic & diplomatic aspects. I never played STW or MTW, but RTW had the same problem, only once the BI hordes were introduced did I fight my first losing seige battle.

But, I'm still waiting for the Mongol & Timurid invasions... I'm hopefully those will pose some challenging defensive battles. Have to wait and see, I guess!