Reducing JPEG file size

[IDKWatMNShouldBe]

Hello, I am designing a YouTube layout for someone.

Youtube says the only support around 250 kb I think, but I've tested it and it's around 450 kb.

Everytime I try to render my photo on Paint.NET on 100 quality, the maximum, it always goes around 800 kb.

And if I do 94, it goes around 300 kb. So how is it that 94 is almost 100, however the file size for 100 is almost twice as much?

Although 94 isn't bad, it still looks a little pixely. Is there a way to reduce the file size without reducing the quaity of the photo?

And I can't crop out anything or change the resolution, I need it the way it is.

Thanks. =)