When you're comparing tanks versus bins (or even just 2 different types of enclosures), you want to compare square footage (floor area), not total volume.
So yes, a 20L is smaller than a 30L because a 20L is only 30"x12" whereas a 30L (same as a 30 gallon breeder) is 36"x18". A CB70-size tub which most breeders use for their adult bps is 33.5"x18.5". So the CB70 tub is only a few inches shy of a 30L.
There are 2 types of 40 gallons... The 40 gallon breeder is the same as the 30L except it is 6 inches taller. A 40L is 48"x12"x16". A 40L has more floor space than a 40 breeder. I know, quite confusing with these idiot names.
But, generally, you want to use 30L for adult bp's, or a CB70 bin or any other tank/bin that has around the same floor space. The thing with caresheets is sometimes they forget that they're talking to newbies that are not familiar with aquariums besides the ones used for fish. So, when they talk sizes, they tend to forget to mention that we are talking about the LONG tanks that are designed for reptiles and not fish.