There's something called 'battle ensign'... It's use is strictly regulated in every navy and I bet PLAN has it's own which I haven't read. It's different from a smaller size national flag used on other occasions on the mainmast.
But I'm not dismissing that the size increased. But there's no point in doing so from the PLAN viewpoint...
I think looking more impressive would be a valid reason to increase the size of the flag; but yes, for all we know those flags could be battle ensigns -- however either way, we've never seen a national flag that large. I've viewed a lot of photos of Chinese Navy surface combatants over the years, and you'll have to take my word that no photographed ship in the past have been seen with such a large national flag, regardless of its role.
And for posterity's sake, the size of the flag we see on those two ships are still quite a bit smaller than the US flag which USN ships can be seen with.
I'm not sure why you're skeptical at the idea that the Chinese Navy have increased the size of at least one version of the national flag they're flying. Regardless of whether it's meant to be a battle ensign or a formal national flag, the fact is those are the largest wuxing hongqis we've seen on any Chinese Navy ship to date.