The first picture is hard to judge. Although it is embarrassing, I'll post the picture below to show why. In my defense, it was my first time ever cantering, and Trooper's first time in a year, and this photo was taken in our second try...
In that photo, my rump is pinned against the rear of the saddle while the poleys are right at my thigh - so the saddle seems too small. In fact, it is a combination of Trooper being off-balance, and my having less than 60 seconds of experience cantering.
I rode that same saddle today on Cowboy, who decided to buck with my daughter-in-law and then tried it with me. We did a lot of laps at a canter or gallop, and the saddle fit fine. I couldn't afford to go any smaller, and the Aussie saddle I normally ride is an inch bigger - but the problems in the photo above are rider & horse, not saddle.
Based on the video, I'd guess the saddle may be too small, but I'd try lengthening the stirrups first.
I've only cantered on 3 horses, so my advice has pretty limited value. FWIW, each horse canters very differently. Trooper has a very short back and a rough canter stride. If I get behind his balance, the rear of the saddle rises vigorously, shoves my rump, and puts me further off-balance. This is particularly true when his weight is forward.
The extra inch between the Aussie saddle above and the one I normally ride helps. It just FEELS better. And saddles should make doing the right thing easy, not hard. If possible, borrow before buying. If it isn't possible, then an extra inch probably wouldn't hurt - if it is within your budget.
Hopefully, no one will follow my example and end up with 5 saddles of varying size & style in quest of the perfect saddle...
Quick edit: sliding seatbones. When learning, a good piece of advice was to polish the saddle with my rump. It is NOT the right way to ride a canter, but it helped my visualize the motion I needed. After a few attempts, my rump stopped sliding...well, except for if/when I get out of synch with my horse. Unfortunately, that is the sort of thing that still happens to me at times. But the 'polishing', while excessive, gave me an idea of what to do. In my first attempt, I was trying to sit the canter the way I would sit a trot, and that didn't go so well! I needed to be more pro-active.