It's not a 100kb model. It's 100kb config files for a several GB model. A small trained layer to stick on top of the real model for fine tuning.
Yes, although it is decently interesting that a model can be fine tuned by just tweaking a small number of weights and training for just a few minutes
When you train a model with new inputs to fine tune you can save the weights that got changed to a separate file instead of the main file.
In other words one can see the small tuning models as selectively to be applied updates/patches.