For rides like TSM, Soarin', Space Mtn., EE, and maybe a few more, for the most part, that is the case.
I'm not talking about, say, 8AM EMH, mid-Jan.
I'm talking in general daily terms.
Actually.. it's not..
The rides as designed have a theoretical maximum capacity - commonly referred to as the THRC - Theoretical Hourly Rider Count. This is different than the OHRC - the Operational Hourly Rider Count.
Those two numbers are never the same. Why? Because the THRC works off mathetical calculations that include load/unload that spans an hour and averages the result (the THRC may calculate a 40 person vehicle unloading "on the hour" and thus gives each hour 20 guests equally, whereas in reality it unloads 5 seconds before or after giving the full 40 to a particular hour) and guests don't fill every seat nor do CMs and sometime rides get delayed or don't dispatch perfectly.
Heck, in a theater with bench seats it could be that there's bigger butts than ancitipated (or lots of little ones even.)
OHRC can be both above or below THRC.
The gap between the two numbers is known as Utilization. It's a percentage - the percentage of THRC actually achieved via the OHRC. Disney considers rides with very low utilization levels to be strains on their operational budgets. They do, already, change the number of operational units (and thereby change the THRC) to match predicted crowd levels. This allows them to do maintenance work on vehicles, staff lower, and etc..
Most notably you'll see them not running all the cars at the Speedway in MK, more duplicitous you -might- notice that Star Tours is only loading 4 of the available six vehicles despite the line being an hour long. Disney considers some waits to be acceptable.
At the same time, they have (as i mentioned above) increased the THRC of several attractions over the last 12-24 months in anticipation of the FP+ system. Minor tweaks add a few more guests per hour. Flumes chug water slightly faster, complex ride systems are reprogrammed to move a little quicker, etc.. They know they need the capacity with more people expecting to get on right-away via FP+.
They have also increased the number of FP's allowed at attractions from the previously set ratios. It used to be set to a near walk-on at all times.. it's now an anticipated and "acceptable" 5-10min wait, with 15mins not be unheard of on a normal day.