See below for park sizes:
MK 107 acres.
HS 135 acres.
IOA 110 acres.
US 125 acres.
IOA is laid out very similar to MK in that it is a loop and is about the same size acreage wise. One loop around at IOA does seem much shorter than one loop at MK. This is because the path at IOA is more circular and more direct than at MK. However, you can't cut across the middle of it it like you can at MK, so back tracking can end up making you feel like you've walked the same as MK or more. Also, many of the attractions seem like they are spread farther apart than at MK which always makes me feel like I'm walking longer when we go farther at one time.
US is similar in size and layout to Hollywood Studios. The attractions are also closer together which helps to make you feel like you aren't walking as much.
Of the two, IOA seems larger with more walking despite having less acreage.
In general, I feel like I walk about the same as when I visit the two Disney parks listed above. If you factor in that we park hop a lot, leave and visit Citywalk for meals, and make trips to and from our resort hotel, we actually walk much more at Universal than at Disney. At Disney, a trip back to the hotel or to DTD for a meal would take a bus or car ride, but at Universal, it is usually only a quick 5 to 15 minute walk, so we do it often.
If only to to minimize the walking, you might want to reconsider the "hoppers". One of the best ways to minimize walking is taking the train at the back of the park to the other park. The walk to the back of both parks is the longest walk. If you are already back there, you could switch to the other park and hit all the stuff in the back of the second park as well. This way you would only have to walk to the back once, instead of twice. You could do half of IOA, take the train across to US, hit the stuff in the back, take the train back, and finish out IOA. Then, the next day do anything toward the front of IOA you missed, and only the front of US.
The train does help going back and forth.
