What's up with the $20 bill being sought after by counterfeiters the most when its face value is shit in comparison to $100? Stores being scammed with fake cash that a pen is needed to distinguish between genuine and fake, its a problem they encounter quite a lot. US Dollars are printed with a single layer consisting of linen and cotton, which is easy for counterfeiters to exploit (like bleaching a $5 bill imprinting $100).
That's something you don't hear about as much with Swiss Francs as they have top notch security features (they're printed vertically at different sizes). Every denomination (from 10-1000) features a globe with security ink showing dynamic color changes and a shifting arc when tilted. The bills are printed using an innovative three-layer composite material known as Durasafe which involves 8 complex printing processes.
$20 dollar bills are the most common denomination in retail, and ATMs here exclusively hand out 20s. Even with prices going through the roof, you hardly ever see higher denomination bills out in the wild. A shop owner may look at you funny if you hand over a $100 to pay for something (and would definitely get the pen out), but would probably not blink twice when handed five $20s, even though counterfitters target them just as much. (Also note that we no longer make any bills over $100 at all, so your only choices over $20 are $50 and $100...)
We do have some currency modernization programs underway, but they will not effect the size of the bills. We seem to think it would be a disaster if our currency were like every other country's currency. We need to think we're Exceptional, I guess.