Do you understand the word "proportionate"?
The difference in quantity is simply a different quantity, no proprotion. The "proprotion" is the comparison of the ratios of quantities to effects for different size quantities (or effects). A large quantity of AN going off had a large effect (to wit: moving buildings 15 miles away), while a medium quantity of AN going off had a medium effect (to wit: partial damage to immediately adjacent building, and broken windows within a mile). The damage at OKC was substantial, yes, yet it was proportionately less than a significantly larger quantity at TC: the truck was parked about 15 feet from the building, which suffered as much or more damage from gravity (having support columns blown out at the base) and was not wholly destroyed, vs. TC where the AN was hundreds of yards away from the nearest structures (which were demolished), and rattled buildings 15 miles away.
You really need to review the meaning of "proprotionate". There's more to it than comparing two numbers: it actually compares four.
IMO you're being contrary, not asking for reasoning to the contrary!
Let me ask it a different way then:
Damage at TC Damage at OKC
------------- = -------------
~2000 tons AN ~2 tons AN
So what's wrong with this equation?