It would be similar to breeding "teacup" bettas, a la teacup dogs. They will have a phenomenal number of health issues regarding their size. Smaller body means smaller organs. You would have to constantly feed them BBS or microworms and would never be able to switch them to pellets unless you take the effort to crush it into a fine dust.
They're already fairly small fish, why would you want smaller ones? Because it's cute
? Teacup dogs are "cute" too but I'm sure you know the controversy behind them.
I would personally try to maximize the health of a regular sized betta rather than experiment with lines just to get a dwarf betta or something. As Becky pointed out, they are culled for a reason, unfortunately :/
Also RE: I just checked, my CT is barely over one inch
I hope he'll grow eventually