I'm not very familiar with the 7xxx series. Looking around, I see both threaded and threadless forks on that model. The drivertains and wheels could be switched, but I doubt that was the case. Your frame, fork, headset, stem, h-bar and seatpost are very different from the 7.1 and not interchangeable. Their wheels and drivetrain components are very similar, though one may be Shimano and the other SRAM. Though the 7100 is technically a 'comfort' bike, and the 7.1 fx is a hybrid, they share much in common. If you're having difficulty getting everything dialed in, I'd visit the LBS (maybe Trek dealer for you) and ask for some assistance is the adjustments. This allows a lot of latitude in adjustments. The 7100 has a threaded headset with an adjustable angle quill stem. Or you may be holding the h-bar with a death grip.

New riders or those that are getting back to riding often keep their arms too stiff and tense, which stresses and tires the arms and hands. The other is that you're not accustomed to riding. But don't attempt this if you doubt your ability to ride 'no handed' even momentarily. If your saddle angle is correct, you should be able to release your hands from the grips without feeling like you're being pushed forward off the saddle. One is that your saddle is angled slightly nose-down, which is pushing your weight forward onto your hands. Given the information in your post, I suspect one (or both) of two things are the primary contributers to your arm fatigue. The saddle adjustment is the first adjustment, and all other adjustments happen around it. But once you have the saddle position dialed in, don't adjust it to compensate for your reach to the h-bar. A longer ride may show that you still need to tweak the angle a couple degrees or slide the saddle back or forward a few millimeters. It's common to set the saddle too low initially, which can hurt the knees and limit your power input. It may take a few short spins around the block before you get the saddle position figured out. There are many write-ups online that show you in detail how to do this. Then set the height and fore/aft according to the angles and position of your knees during the crank rotation. Set the angle so the saddle is initially level or slightly nose up. It's a good idea to set the saddle angle, saddle height, and fore/aft position in that order. Your saddle adjustment is the first adjustment you make on the bike. A larger frame will have a longer top tube, but it's easier to get your bars higher with the larger frame. Depending on your leg/torso/arm proportions, one may be more comfortable than the other.

: At 6-1, you're probably right between the 20' and 22.5' sizes. Be certain yours is correct for your body proportions. First, do you know that you have the correct size frame? There is generous room for adjustment on most hybrids & comfort bikes, but there are only four frame size choices.

Trek bike 7100