There should be less ports overall in the game and anything less than a size 3 port should be removed unless it's the only port available for a country. It takes some major shipyard facilities to repair even the smaller warships so having the capability at all ports to repair battleships and land an invasion force is strange to me. I'm reasonably familiar with UK and if you tried to funnel an invasion through some of the ports you'd have ships queued up for a fortnight trying to dock. For example, Pembroke, Barrow, Bournemouth, Aberdeen, Inverness (arguably Lowestoft, Grimsby and Edinburgh also) are all really mainly just decent-good sized fishing ports. You need some way to simulate the Mulberry harbours though, which if provided, really means a viable invasion can be launched anywhere on the coast.
I think the main issue is strategic redeployment is too quick and being able to deploy directly into combat without any negatives to combat, as mentioned in another thread, means the player doesn't really NEED to have a credible defence for the french coast.
If you want to compare to real life, quote courtesy of wikipedia, then "By D-Day 157 German divisions were stationed in the Soviet Union, 6 in Finland, 12 in Norway, 6 in Denmark, 9 in Germany, 21 in the Balkans, 26 in Italy and 59 in France, Belgium and the Netherlands." Some, espcially in the east, were badly understrength. 59 divisions under Von Runstedt was 850k men, works out about 14.5k/division so not a million miles away from 10k effective fighting men in a HOI 3+1 div structure (allowing 4.5k for support troops, cooks, drivers, clerks etc). The armoured reserve for Von Rundstedt was circa 13 Armoured divs, I assume that means 46 infantry divs remained. There are 28 provinces to Brest and a further 15 to Spain, so 43 provinces in all. That would, incidentally, mean a HOI div on every province from germany to spain with 13 ARM div in reserve.
Imagine doing Barbarossa with 50% of your german troops based somewhere other than the Russian border.