It is generally understood that interior volume in the JSF is at a premium because everything from fuel to ord to electronics needs to be carried internally to retain signature.
"Generally Understood?" Is that the same as "It is rumored that...." But I get what you mean and agree: growth margin is always an issue and the JSF is no exception. And if meeting the interior volume specs for the JSF means having no interior volume growth margin, then that's the fault of the government requirements folks, not the contractor. If memory serves, the Lightning, Starfighter, Harrier, Skyhawk, Tiger, Saber, Super Saber, Dragon Lady and many others had that problem. And one of the main reasons the Super Hornet exists is because the Classic Hornet ran out of growth margin.