You start a ride with some food in your belly, some glycogen stores, long term storage (fat), and mostly comprised of water. During the ride, you sweat out some water and trace weight of minerals. Everything else, matter wise, is still in your body, just in different composition.* You replenish the water during and after the ride for an overall net zero. If you drink more water than lost, net weight gain. Then you eat some food. Net gain in weight.

Now the good news is the food in your belly and glycogen stores were depleted i.e. chemically altered, and will eventually exit your body as waste. Perhaps a bit of fat as well, if your body had to dig that deep for fuel. The post ride fuel will restore glycogen stores and, if you didn't eat more than necessary, will not be put in long term storage.

*If there are any physicists out there perhaps you can comment on whether some miniscule amount of matter is converted to energy. That might be fun.