Box Fill for Line Voltage Thermostat
Does the box fill calculation account for a device like this differently?


No. The Code does not distinguish between particular devices.

If the device is yoke mounted it requires a double volume allowance of the largest conductor attached to it in the box.

Here is the pertinent 2014/2017 National Electrical Code Article:


  314.16(B)(4) Device or Equipment Fill.  For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table 314.16(B) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap. A device or utilization equipment wider than a single 50 mm (2 in.) device box as described in Table 314.16(A) shall have double volume allowances provided for each gang required for mounting.


If the device was surface mounted (like the old bi-metallic style probably was) on the outside of the box, or is not technically "yoke mounted" then this requirement does not apply.

Your conductor count looks good. According to Table 314.16(A) the deepest 3" x 2" device box is only good for 18.0 cubic inches or 8-#12 wire count.

So, to be Code compliant, you would need to remove the pass-thru conductors and make those joints elsewhere, or change the box to a 4" x 1 1\2" square with a single gang plaster ring, or they make some single gang boxes with an expansion on the side that may have enough volume.