I don't know what happens as the day wears on but I assume they straighten because by the next morning they have resumed their normal/natural posture and they stay that way for the duration of the flower.
Regarding pod size relative to primary and secondary buds, yes, there is a difference. As you thought, a primary pod is a good 1/3rd larger than a secondary. With the exercise in the other post, I could just as easily use a secondary since it's more or less an exploration and I only need a few seeds to find out if I like the cross. Taking it a step further, if I like the cross, then I'll use Primary pods to produce lots of seeds and then 'carpet bomb' several pots with many seeds.
