Hello.
I would not worry about the size difference as long as the smaller fish is eating enough and not being bullied bad or cornering. There is usually a smaller fish and of course a large one. Nothing to worry about. Some bulling is normal especially when new fish are first put together
What I do think is a potential big issue is your ph fluctuations. Especially since it going from alkaline to acidic. I would try hard to get that stable. This is very stressful on your fish. Keeping discus in a planted tank can be challenging at best but especially with major ph swings. As you probably already know discus are a tough and hardy fish only when they have clean and stable water.
Nice looking fish!
Good luck.
Respectfully, Matt