Disney has gotten really good, to an almost evil degree, in controlling crowds. During times of lower expected guests, they will announce cheaper tickets (like for kids), or a so-cal deal. As others have said, they also lower staffing on slower days, and don’t run as many vehicles on attractions. So even if the park has less people, the wait times tend to stay higher than you would think.
It’s not like knotts berry farm or even universal studios where there CAN be really dead days.
The only other thing I can add is to avoid Mondays and to a lesser extent Tuesdays, they tend to be the worst. Weekends and Wednesday and Thursdays tend to be best choices. I have been there where Sunday was the least busy day.