That's strange, with settings like the ones below, I can go from 60kg to 120kg in a few seconds.
Is the blue weight number in the top left below the bar not going up after digesting enemies?
So, if you've got the Maintenance Calories extremely low when compared to the Maximum Calories, you'll very quickly gain weight while digesting enemies, but you'll also very quickly return to Maintenance Calories. Meaning you'll quickly start losing weight again.
There's a kind of scaling in effect that the further you are from your Maintenance Calories, the more quickly it'll change to try and get to that Maintenance level.
So very low Maintenance makes it very volatile, quickly gain and quickly lose calories.
If you want your weight to stick around, I'd recommend more balanced settings.
I personally use settings that make the bar look like this: