I have a Ficus that I received from my Grandpa four years ago as a cutting. It's grown in cycles, but in between growths, it'll drop most of the leaves. It went from close to thirty leaves down to three for a few months and then grew back. For the last two months or so, during prime growing season, it hasn't put on a single leaf. I've been careful to not overwater, it's getting plenty of sunlight, and in the last two days it's lost two leaves. Does anyone know what this leaf coloring may mean?

To be honest, I would recommend going to a specialty forum. Not many people grow ficus as bonsai.
In general they like heat, sun and water. So if you have had it outside, watered it well and you have a good summer it should be exploding with growth. (If I look at this plant it looks like something that has stood inside, has received too little water and may have had some visitors like scale or spider mites)

Is it possible that the pot is too big for the tree? I know that as a general (horticultural) rule, potting into a container/pot too big for the plant can cause problems with growth. The plant uses more energy developing a root system rather than flowers or foliage.

Not sure if this holds true for bonsai.

Also, might it be worth inspecting the roots for any root-rot or pests? Ficus is quite resilient so it should be thriving, or at least growing, all things being equal. I had a sorry looking tree that was on deaths’s door after my inexperienced hands kept pruning it (it was my first proper tree after many years). Even though it only had a handful of oversized leaves, it kept going and looks like a different tree today.