more cum sign of growing up?

As you grow, your capacity for storing semen increases, so that might be likely.

Also, if you didn't ejaculate prior to that for a long time, you would have stored up more semen, and would have released it.

it could be a sign, but it's usually more gradual. The cause is more likely from other things. When you had sex, did you stay sexually aroused for more time than you usually do (did you have sex for a longer amount of time), also, was there a greater period of time in between the last time you came and the time you had sex? Getting more aroused and staying aroused for a longer amount of time almost always results in increase load of semen.
No, your body does not store semen. You have glands that secrete all the fluids to make the semen just before orgasm, and it has been found that longer and stronger arousal makes these glands inject more fluids into the seminal flow due to the increase stimulation.
Other than that, it is common to just have a randomly large load for no apparent reason, or a small one. There is no doubt in my mind, though, that since you are 16, you're sex organs are still maturing.