Oh, no that does not work because ships scale up in three dimensions; so, bubbles need to scale up in 3 dimensions otherwise you allow spaghetti ships in the high mass ranges. Think of it like this: a 100m cube has a volume of 1,000,000m^3, but a 200m cube has a volume of 8,000,000m^3. So doubling your shield bubble radius gives you 8 times the coverage; and should therefore cost 8 times as many chargers.
Another way to look at it is if you make a charger that is 3x3x3 blocks, it should take a charger that is 6x6x6 blocks to double its radius.
Also, as far as I can tell, there is a bit of "free" radius that the system gives you so that really small shields can still cover something meaningful, which may contribute to apparent fall off, but this is just a flat bonus that becomes statistically insignificant on bigger ships, not some aggregating penalty.