After about two years of use, I'm convinced that on the current models of Herman Miller's Eames Lounge Chairs, the seat cushion is overstuffed to the extent that the ergonomics of the chair is compromised. The sitting position is too high, leading to misalignment relative to the lumbar cushion.
The cushion appears to be a single piece of foam, wrapped in upholstery. Can I remove the foam, trim off a couple of inches from the bottom with a serrated knife, and replace it?
You'll have to look at your chair to determine how the outer fabric* is attached, then figure out how to remove it without damaging the visible material.
Once the fabric has been removed, you may find that the seat cushion is glued to the bottom of the seat pan. In that case, you'll need to carefully remove foam from the seating surface while retaining the contours. If you don't do this job well, you'll end up with a lumpy surface that you will probably dislike more than the initial problem.
If the seating foam is easily removable, you should be able to slice a chunk off the bottom without issue. A serrated knife may work, but a hot wire knife will probably work better. You can buy a commercially made one, but for one time use, the internet is full of how to build a homemade one videos and text descriptions. PLEASE use common sense when reviewing the instructions to decide which one you want to build - some of them will be quite safe in their design, others may be outright dangerous. If you're not sure, feel free to ask another question about it here.
You mention removing a couple of inches, I'd suggest that you may want to remove smaller amounts in repeated rounds until you get to the cushion height you're after. It's far easier to remove a bit more than it is to put some back. On the other hand, seat cushion foam is readily available in flat sheets at many local retailers (you wouldn't even have to go to a specialty upholstery shop), so you might consider removing a large amount then building back up with thin layers of new foam until you get to the desired thickness.
Once you've managed to slice off enough to get comfy, you'll want to put the fabric back on. Lay it back over the cushion, put a tack/staple in the middle of one side, stretch it to the other side and tack it there. Do the same for front & back, then work your way around, stretching the fabric to keep the fit nice and tight. Since the foam is now smaller than it was, you'll have some extra fabric, so after reattaching it, you'll need to cut off the excess.
If, by some chance, your seat cushion fabric has a zipper so that you unzip it, then slide it off the seat pan to remove it, you're going to have a very easy time getting the fabric back on, but a very difficult time getting it to fit right. You may need to take it to a tailor shop to get the fabric resized to fit the new, slimmer seat cushion.
Again, the internet is full of videos on how to upholster furniture, so I'll leave it at that simple description and suggest you do some more research on your own.
It may be easier to find a new chair that fits your body better, or at least has an adjustable lumbar support. (I always adjust the lumbar support all the way out - it may be bad for my posture, but I hate 'em.)
*For simplicity, I'm using the word "fabric" to refer to the seating surface, whether it's fabric, leather, pleather, or any other type of material that the seat of your pants contact when sitting on the chair.
