Why does the PS5 change the screen size sometimes

According to this article:

Sony is swapping the ‘X’ and circle buttons on the Japanese PlayStation 5 console, shifting towards the West’s use of the ‘X’ button to confirm actions and a circle to cancel.

The original source for this article was from Famitsu (translated to English):

As I learned from this interview, the so-called decision button was not 〇, but × was the default setting. According to the SIE public relations, this is globally unified including the Asian region including Japan, solving the problem that the operation is different between the game of × decision and the system menu of ○ decision, and the occurrence of erroneous input etc. It is said that there is an intention to prevent it.

Multiple sources, including this source state that for PS5 games, the traditional layout for Japanese users will remain the same, thus the circle and X options are only swapped in regards to the PS5 hardware:

Perhaps even more confusingly, the Japanese versions of PS5 games will still use the previous definitions of X and Circle, separate and contradictory to the PS5 hardware itself.

So, it seems that when dealing with menus specific to PS5 hardware, X and circle are swapped (X to confirm, circle to cancel), but in games, they are what Japan has been using for decades (X to cancel, circle to confirm).

Unfortunately, there isn't any granular ways to forward / rewind video clips using the PS5 Media Player. The only option available is the +/- 10 second using the left / right arrows on the D-Pad, which you've mentioned.

However, when watching DVDs / Blu-ray movies, expanded controls are available, including 1.5x playback and up to 30x fast-forward.