Most likely because you are playing on
full AI aggressiveness
and
full Difficulty
which is giving you a hard time. I am not quite familiar with this game, but most give the AI certain 'perks' based upon the difficulty level (not necessarily bonus', but rather they don't experience certain penalties the player does). The aggressiveness rate could be causing the large concentration of forces that you are experiencing. The AI will not attack with small individual units, but would concentrate, then attack. Given the high aggression of the AI, its goal is to concentrate as quickly as possible to get a force large enough to strike. A more passive AI will concentrate on holding the line, vs. concentrating in an offensive.
There are many experience work-arounds when dealing with the AI. Frankly, the best encounters are when you know very little of the AI's behaviour, and are blundering through. Eventually, you will read the patterns of the AI, and see how to take advantage of their concentrated armies (i.e., to encircle and cut them off as they do not protect their lines of communication).
I believe that you have to concentrate your artillery, and there are probably certain leaders with specific benefits, for attacking a fortified strongpoint.
If you want to figure out the AI, start at an easier setting, then learn how the AI operates (so you can out rock-paper-scissors-lizard-spock it).
One to one, the German army really was not 'better' than their French counterpart, it just happened to be more concentrated and caught the French high-command off-guard. Watching the French and British early battles in the north (BEF and 5e Armee), they did fairly well in lieu of being massively outnumbered. The issue with Belgium, is that historically they were dispersed too much, and retreated to Antwerp, leaving the route to France opened up.
You may want to try a different plan at max difficulty and aggression. Try an Eastern focus for Germany, as the space to out-manoever against Russia is greater than in France (get bogged down their quicker). A defensive west, and an offensive east, may be better at this level of difficulty and aggression (allow the French to bash against your fortressess instead, while you are free to manoever in the East).