An extra response, which I hope is of additional help.
https://www.schilkemusic.com/products/mouthpieces/tuba/The Geib model has the same inside diameter as the Helleberg SH-II with a modified rim contour. The Geib cup is more bowl-shaped and slightly shallower than the SH-II. The entrance to the throat is tighter yet it shares the same throat size as the SH-II with a standard backbore.
If the OP likes the size then that’s that, it suits them. The Eb Tubas that I’ve played haven’t really shown me particular practical benefit from using large cups and I switch about just either side of a Wick 3 (31.25 mm) on a four valve instrument. Played out on a Wick 2 today but using a Bach 25 helped me get my first top F. In contrast the bigger cup has helped me develop better tone and low range, but it’s all marginal gains rather that clearly different.
I think that the deeper and bowl like cups help with tone but hinder flexibly - it’s all a trade-off.
The Wick range are what they are and mostly that’s pretty good, and they do small shank versions of their standard range.
The JK 8C mentioned by MA above isn’t very large at all (30.5mm) but it still does the job and does it for a Tuba with more than three valves.
https://www.thomann.de/gb/jk_exclusive_ ... _8c.htmThe