7mm-08 in the 100gr range travels a good bit faster than a .243 in that same weight. 100gr .243 and 120gr 7mm-08 travel about the same speed.
For various reasons you are correct, a 100gr 7mm is about 100fps faster than a 100gr .243. But that's neither here nor there for you are throwing out about every other ballistic principle. There are lot's of good reasons no one makes light for caliber "big game" bullets and you can't compare them against standard/heavier for caliber bullets. BC, SD, bullet integrity are all thrown out the window if you do that. For the record, when you do that the 100gr .243 would pretty well smoke the 100gr 7mm in performance on big game despite the 100fps at the muzzle.
- 100gr .243 Nosler Partition, approx FPS (muzzle) = 3000, SD = .242
- 140 gr 7mm Nosler Partition, approx FPS = 2750, SD = .248
Similar bullet design, similar SD, 250 fps advantage to the .243. That's a comparison that means something. Given equal expansion that .243 should penetrate more than the 7mm, wound channel should be very compable.
(For the record approx SD for 100gr 7mm is .177 and approx for the 120's is .213. Worse potential penetration given equal expansion)