The guage isnt that different to weichs
Weichs vs solos (in millimetres):
IV.....2.8....2.6
III....2.0....2.0
II.....1.6....1.6
I......1.2....1.1
But I think they "feel" thinner because they feel softer and theyre easier to push down ie lower tension. (when tuned to orchestra pitch)
Volume wise, the fundamental note is quieter, but the string is more complex, and the bass resonates more with the lower tension (your results may vary obviously).
All-round these are a different string to weichs and mittels. They are brighter and more complex, while having that spiro character.
You may also have to change your bowing approach to get the most power and tone out of them, otherwise they can sound weak, with a hollow metallic tone. So you have to give them a thorough test to know if they work for you.