How can I get offshoot small amaryllis bulbs to get large?
They can take a few seasons to fatten up, naturally, but you can take them small and separate them into their own space, and they will continue to grow like this as well. Sometimes I am a little too rough with mine when repotting, so they break off. I just stick these into their own container, and they keep on going!
You can also try fertilizing with something a little higher in potassium and phosphorus, as this can help them fatten up a bit as well.
Here is an article to refresh you on the care of amaryllis: https://www.gardeningknowhow.com/ornamental/bulbs/amaryllis-hippeastrum/amaryllis-care-instructions-how-to-care-for-an-amaryllis.htm