In an industry where "too big to fail" is a real thing, taxing bigger companies more seems perfectly reasonable. Not just a little bit - significantly more, even painfully more. Create some actual incentive for banks to not get that big.
In some businesses there is also a benefit in scale (like in trading), and there you see the US banks leading because they've had it.
If you want the US banks to fully take over IB and corporate banking, definitely do tax the EU banks painfully more.
Or tax every financial counterpart painfully more, so then eventually the customers pay much more for their loans because the financial system has become so expensive.
The world of money existed before banks, it will exist after.
In short, due to international capital sufficiency regulations, there's now a market for excess regulatory capital. The attempt to prune low performing assets is due to a market as an alternative to low return on capital investments.
It's not because of charges and overhead.
Banks aren't worried about bad assets more than usual, they just have another arena to make money in now.