My honest opinion is it was a HUGE first step but must be followed with baby steps. I think you may have opened up the possibility to her never realizing the pleasures of a larger guy and to agree to a threesome when it was never an option before is majorly huge. Just be careful and don’t just make it about sex or you could lose her, keep it intimate between you two, by not just sex but by talking honestly and openly. let it progress slowly and it will be much more rewarding