Sounds like you're using a lot of whole wheat.
This is the skinny according to Hobart:
"The ingredient percentages are based on a flour content of 100 percent to simplify using the recipes to make various sized batches and compute the moisture absorption ratio. The heavy bread dough recipe above is a criterion listed in Section 4.4.2 of Federal Specification 00-M-0038K, Standard for Electric Food Mixing Machines.
The flour used in the recipes is hard wheat flour, enriched and bleached. It
contains 11 to 12 percent protein content and a 12 percent moisture content.
Flour which has a lower moisture content will decrease the moisture absorption, cause difficulty in proper gluten hydration, and make a heavier load on your mixer. Flours containing high quality protein, such as high gluten flour, result in a dough which can be very difficult to mix. If high gluten flour is used, reduce the batch quantity noted on the Mixer Capacity Chart by 10 percent to prevent overloading of the mixer.
The temperature of the water used in the recipes is 65°F to 75°F. Colder water temperature will cause the dough to be harder to mix. If you plan to mix doughs using cold water, the batch size may need to be reduced by 15 to 20%.
Also considered and noted on the Mixer Capacity Chart is the moisture absorption ratio (AR). This is the ratio of the weight of water to the weight of flour expressed as a percentage.
AR = weight of water ÷ weight of flour <-> 100%
The absorption ratio gives an indication of the relative "heaviness" or "wetness" of a batch. In fact, the capacities listed on the Mixer Capacity Chart for the products listed below are based on the following absorption ratios:
PRODUCT ABSORPTION RATIO
Heavy Bread Dough 55%
Medium Bread Dough 60%
Light Bread Dough 65%
Thin Pizza Dough 40%
Medium Pizza Dough 50%
Thick Pizza Dough 60%
Raised Donut Dough 65%
Whole Wheat Dough 70%"
Between my use of some whole wheat, high gluten and durum, and my cold mixing (use of refrigerated poolish) it's seems prudent to limit my dough batches to 12 cups (75% of KA's 16 cup recommended maximum).
A more complete rendering of Hobart's instructions can be seen at-