Different size of the same system in same layout

Hope I can describe my problem:
I’m preparing a book with several playing scores (Brass). To avoid impossible page turns, I decided in some cases to make a small score an two score excerpts as separate layout frames (one for trumpets and one for trombones). Here I can change the size of the whole score for the conductor and the parts stay in the orginal size. So far everything works very well.
Now I have a special case: a piece which is a just bit to long and I want to have a score where the trumpet parts are normal size and the trombones have a smaller size, so that the trumpets and the conductor can use this layout. The trombones should get a separate layout, where I only have the three trombone parts (normal size). Because I changed the size of the trombone systems in the score to 75%, they now appear in the layout frame in the same small size. Of course I could just copy the music to the new frame, but than the connection between the trombone parts is broken and I have to be very careful when I have to make some changes later.

Hope this is clear and someone has got a clever idea!

Rather than using the option to scale the players’ size via Edit > Staff Size, instead put a system break at the start of the music frame containing only the three trombone staves, and then override the staff size in the Properties panel.

A bit of mathematics, but it works. Thank you, Daniel!