The smaller the thread number...the thicker the thread......thus the BIGGER needle (higher number) you need in your machine.
It took me years to learn that also. I just learned that most 50wt threads have three strands of thread, but some of the newer 50 wt have two strands of thread.
I took a class at the Manchester, NH quilt show and learned SO MUCH I never knew about threads....and needles. It has made my sewing life so much easier.