Bro, those are different angles. If I shoot myself from the knee up I could look 6'6".
Shoot me 20 feet above ground, in certain angles I could easily look like a ****** even if I wore spandex.
And the second pic, the jeans look pretty slim, see how close those jeans are to his thighs? There are even pulling there, it isn't straight cut at all. That is like reverse taper, it becomes slimmer from bottom to the top while the jeans in discussion is just square and straight. Plus the flaring is from front to back instead of side to side so again it makes it look slimmer. it's also basic that shape of angles makes you look taller/slimmer. A triangle shape/silhouette makes you look taller than a square one.
I hope Nii doesn't me using him as an example. Nii is pretty proportionate in his suit, full bodie suit with very little taken in in the body and straight cut pants (might be slight taper). Then in jeans, it's tapered much heavier and it makes him look taller, his legs looks longer and he does look slimmer.