From my experience the relationship is about 4 to 1. Meaning for every 4 units of strenght gained there will be 1 unit of muscle. Ofcourse the scenario with the units is all hypothetical.
But after years of training with heavy weights and good form following the motto of "must lift heavy" and do all the basic movements i realized that my gains even with some "supplements" if you know what i mean , were nothing extrordinary. But me strenght was very decent.
Then there came a time where i threw that way of thinking out the window and i just got tired of trying to get strong and made a switch in the routine, and trained solely for the intense burn, and i mean intense, on some sets ding reps as high as 30, instead of the usuall 8-12. Basically doing whatever it took to get a mind blowing burn like i was going to tear the muscle. Needless to say lot's of pain in training. But the results were totally different from my previous routine. While my strenght decreased by about 20 to almost 40% on some lifts i blew up in size. Over the course of about 8 months i went from 240 to 275 and finally looked like a BB. BTW i am 6'2. My arms went from a pumped 17.5 to a full 20 inches, while being much weaker but still by no means weak.
That had conclusively proved to me that strenght and muscle size go hand in hand but in a very disproportionate manner, about 4 to 1. At least for me.