I can't address the OP's issue, but the Di2 shifters are exactly the same as my regular Ultegra ones. I don't have disc brakes on that bike, though, maybe that would feel different? Actually the regular Ultegra ones on my Guru are easier, because they are the short reach brifters that I bought on E Bay as they were no longer made in 2011. But, the size of the shifters are the same, it's just the reach to the brakes.
My Ariel has disc brakes, but flat bars. It has the mid range trigger shifters typical for this kind of bike, but I find them much harder to shift than other ones I've had. I wonder if it's the disc brakes making it feel this way? I thought it was my hands having issues with the flat bars, lifting weights, and being older!