Mice will gain ~1-2g per week on HFD, and remain mostly stable (<1g per week) on Chow. If you see large swings in body weight consider that your weights are likely not accurate. Some failure modes include:

• Using different weighing scales - different scales (even the same model) can be off by ~1g, so if you swap scales between days you will add noise here.
• Check that the scale is tared to 0 between each weighing - if the scale tare "drifts" during your weighing it will introduce systematic errors.
• Scales can be inaccurate if placed in a hood with the air on. Turn the air off while weighing.
• The scale can sometimes touch something like a notebook, giving an improper reading. Make sure there is nothing nudged up against the scale while weighing.
• Mice move around, making it hard to get an accurate weight.

The best way to deal with all of this is to enter weights into a Google Drive sheet as you weigh them, and use formulas to calculate the food eaten as you weigh them - if you get numbers that don't make sense (as in yellow below), you can re-weigh the food right away, or make a note of it if the weighing seems accurate but not plausible.

Example of a weighing scheme in Google Drive