Correct me if Im wrong anybody, the way I see your box fill is that you only have a 2 conductors entering the box and 4 conductors leaving the box for a total of 6 and you add 1 volume for the ground. So 7x2.25= 15.75 cu.in.

Sorry you're right, I'm tried and my math was bad. You're correct I should have said 3 hots and 3 neutrals plus the 1 ground. My second question still stands though regarding the heat and current potential.

When determinimg max box fill, engineers must consider many things. In your case, with no more than 20A passing through the J-box, you could probably (improperly, though!) stuff as many wires in there as you could, and not have a problem. However, what if it was a multi-wire circuit? Or buried under insulation in a hot attic? Remember, ampacity of household wiring is limited solely for heat dissipation. In free air, these same wires can carry much more current than when they are confined in walls and ceilings. Whenever I have a box that is at or near the fill limit, I put on an extension ring. For the $1.99 cost, it's a sensible precaution.

Try and fit a GFCI receptacle into a box within fill rules and see how well it works.

The device allowance is 2 of the largest conductor, so often #12.

It is easy to meet the letter of the fill table and struggle to get all the wires in.

Pinching wires between the back of the GFCI and a metal box can cause strange failures that may not even show up for weeks or months until the insulation slowly deforms enough to fail.

I had a call once for a circuit that was tripping the breaker.

The line feed to a GFCI was pinched so hard against the back of the box it split the insulation after about 3 months.
But it was all to code.

you guys are correct, box fill is only the minimum required by the NEC. I very often go to the next size up depending on how well i can fit the wires or if its a GFCI like "brickeyee" mentioned.