Laboratory conditions strike again!
The problem with this test is that it doesn't appear to replicate the real-world conditions involved. The "bendgate" video showed uneven pressure being applied across a corner, which - unintentionally, I'm sure - is a more realistic representation of what would happen. A phone jammed in a front pocket, as reported at the start of the controversy, would be subjected to unequal pressure on one corner as its owner bent their leg to sit down and stand, or walk around. The problem is magnified if that corner is also the one with the controls, which present a significant weak spot.
The full cross-section of the phone is a great deal stronger than a partial section across the corner. Testing the full cross-section doesn't address the demonstrated failure.
The smaller phone is more robust due to the relative strength of the case material in cross-section. The larger phone is weaker. It's rare, but this bending issue obviously does happen, and it's a case of Apple apparently not realising that scaling the phone up without accounting for the square/cube law is asking for trouble.