The volume of the powder is so minor compared to a gallon as to not matter in practical cases. Don't worry about it.
This small error in concentration is overshadowed by other sources of error:
- Your ability to measure the temperature and then chose the correct developing time. Keep in mind that temperature can change over the few minutes of the developing phase, and that you probably aren't adjusting for it.
- Your ability to expose exactly as intended. Between the metering error, metering the right representative part of the scene, lens f-stop error, and even batch of film, there is probably a good fraction of one f-stop slop.
- The freshness of your developer and how well oxygen was kept from it in storage.
- If you're exposing so that 1/4 f-stop difference high or low actually matters, then you're not exposing right or should be bracketing anyway.
- The development process largely runs to completion so that a little overdevelopment shouldn't make much difference. Generally what happens with significant overdevelopment is that you get what appears to be global fogging, meaning the blank areas get a little density. Even 25% overdevelopment is unlikely to make much difference, and your solution concentration error is way less than that.
Some developers, like Microdol-X for example, did other things than just develop the exposed silver. Microdol-X also dissolved the edges of the grains a little bit in a effort to increase resolution at a slight decrease in contrast. The main danger of overdeveloping there was not so much elevated unexposed density, but excessive grain dissolving, lowering exposed density and decreasing contrast. I haven't done wet-silver processing since the pleistocene, but I don't recall D-76 having this dissolving agent. It was generally considered a good quality black and white negative film developer, again if I remember right.
If you're worried about this, add 5% or so to the developing time. You'd have a hard time measuring the effect of that even with carefully calibrated equipment. There is almost that much slop in when the bottom versus the top of the film is covered by developer when pouring in, and when exactly the development is "stopped" by pouring out the developer and pouring in the stop bath.
Again, don't worry about it.