I think I'd agree with @EricM that the GEN1 does have a lot more curves compared to 2 & 3. Maybe because the GEN1's F150 counter part looked so skinny...I'm trying to see what you're seeing. Pics for comparison:
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I guess the Gen 1 is a bit more protruding, and more rounded, than the Gen 2. Gen 3 looks about the same as Gen 2.
TRX for comparison. Nice big butt, but more of a "plateau" look than the Gen 1's protruding arc.
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And the TRX at certain angles it does have a lot of curves as well but at other angles it looks a lot like a Rebel. It's a lot more noticeable in person though.