What Evelyne says is correct. However, I believe you are also correct if you assume a fixed dough weight. In other words, if you have two dough batches of equal weight and one is more hydrated than the other, the mixer may labor more to mix the dough with the lower hydration, especially if you are near the maximum capacity of the mixer. Usually when you experience this situation in a home setting, you make smaller dough batch sizes so as not to overtax the mixer's motor.
As Evelyne has mentioned, commercial mixers are rated by weight of dough, although the bowl size is what is used to identify a particular mixer (e.g., an 80-qt. Hobart). If you go to the Hobart website, at http://www.hobartcorp.com/hobartg5/pr/products.nsf/pages/food-prep_mixer
, and look at the mixer capacity chart (linked at the bottom of the page), and especially the footnotes, you will see that there are all kinds of limitations on the use of mixers based on type of flour used (high-gluten flours require a 10% reduction in dough batch size), operating speeds, hydration levels, and total dough weight. Evelyne recently discussed the last aspect at the new PMQ Think Tank at http://thinktank.pmq.com/viewtopic.php?p=1257#1257
:A mixer's dough capacity isn't rated on flour weight, but by total weight of flour and moisture. Consequently if you are using 50 pounds of flour and use 25 pounds of water or total liquid (this is 50% moisture rate) your total dough capacity will be 75 pounds. If you use a 60% percent moisture rate, the dough will weigh about 80 pounds.
If you use a wetter dough like 58% plus, then you will get less capacity as the motor must work harder. Usually 60 quart mixers handle about 90 pounds of 50% or just around that, dough. The more moisture in the formula, the smaller the batch recommended.
Go to the Hobart web site, they have the information as to the various capacities, moisture loads etc of all their mixers.
Although some operators have scales to weigh out flour, many just like to use standard 50 lb. bags of flour. Doing so can often exceed the rated capacity of the mixer and put it under a lot of strain once they add the water. The higher the hydration, the greater the mixer has to labor because of the greater weight. Some operators add the flour gradually so as not to push their mixers over the edge.
Another point to keep in mind is that the size of the mixer’s motor is something to consider in making a mixer selection. Two mixers with the same nominal capacities but with different horsepowers will have different recommended maximum dough capacities (the mixer with the more powerful motor will produce a larger dough batch).