It's not about defending anything, the idea is that an axis defensive line is more breakable, that's the whole point. Allied have more cheap units while not being less effective, bigger crews and stronger emplacements plus better balistic weapons. In addition, the sinergy between allied armies both in attack and defense is bigger and lead to more efficient results. It is generaly believed axis field better armor, but the differences aren't that big anymore once british entered into the picture, and the prices for wich axis get their heavy toys are discouraging. Plus, the heavies' limitations. For comparison, is churchill limited on one unit at a time on the map? Well, Tiger/Elefant/JagdTiger are.
I like how you mention bigger crews when it applies to literally ONE army in game.
Twice the amusment considering that except brit 6-pounder every single weapon team is equally or more expensive then axis counterpart, so its axis who actually have more cheap units while not being less effective.