Well in one of the music reading apps I use iRealb the chart is smaller in landscape mode because whatever playlist that song belongs to is listed on the left hand side. In portrait mode it's just the chart and fills the whole screen.
With the regular iPads larger screen the chart is larger. If you had the app on say a iMac it would be even larger. You can use the same app on a iPhone but it's difficult to see. The app first came out for iPhone and that's what we used for digital sheet music in lieu of bringing a laptop.
When the iPad came out it changed the digital sheet music game. The size of the regular iPad is close to a sheet of music paper where the mini and iPhone are not.
With the app ForScore same deal smaller the screen smaller the music. I was wondering if any musicians using a mini for chart reading find it difficult on the mini seeing notes, chord qualities, accents and so on.
I even have a Bluetooth foot pedal for turning pages so your hands never leave your instrument.