Well I mean there's not really one answer lol. There's a reason there are whole seminars and books and DVDs on this stuff. What I've learned is based partially on that, and mostly on spending thousands of hours watching dogs run agility.
And to every rule there is just one exception after another.
There are a lot of poodles in agility but I've never personally seen a Standard who jumps well. Between the short back and the (usually) ramrod straight fronts they just sort of fling themselves over.
Belgians in general seem to jump big and have trouble turning over the bars. Probably the same reason they are so good at the big jumps required in some of the protection sports and dock diving. Many seem really straight and stiff...from pastern to back to front/rear angles.
Retrievers are often front-heavy and bigger boned. Angles vary hugely...from very well angled to imbalanced to straight as sticks. But even the lighter boned retrievers like FCRs tend to be front-heavy for agility, and some have so much front it impedes them. They are bullets in the water but we're talking jumping styles. Then there are very moderate retrievers who tear up a course and compete right up there with the BCs.
GSDs are the opposite extreme. Most are so overangulated for agility and with such soft pasterns and such that is undermines their stability and ability to get off the ground or land safely. Probably 90% of the GSDs I see run preferred (4" lower jump heights) for this reason, and they still struggle.
One of the only breeds I can think of that seems to be as flexible on an agility course as BCs are the Shelties. There are reasons these breeds are so common and it's not just the biddability and such. They have long (but not too long) flexible backs.
And like I said it varies a great deal. And you have to really watch the dog, watch the handler, watch the clock. See the course type. Big open flowy courses will favor open, extended jumping styles. Tight technical courses will favor a dog who really turn over the bar. The best dogs can do both. Which as BB said is a combination of both training and physical ability.
And honestly really good training is going to get you farther than incredible athletic ability,
as long as the dog is sound and reasonably athletic. Because when the dog has the training to be very efficient on course, make every stride count, accel/decel appropriately...they have a huge advantage on course in terms of time, consistency, and longevity of career.
All this is just my opinion of course so take it as you will
And as I said, for every "rule" especially in terms of generalizations of how breeds move on course, there are endless exceptions.