Yeah, typical nickel-brass is only a little bit heavier than brass alloys is not containing nickel, and/but being that nickel-brass a little bit more difficult to dent, we're finding that in Chinese and European instruments only a half millimeter thick - whereas some of the vintage American instruments which feature all brass inside and outside slide tubes - featured tubing that's somewhat thicker than a half millimeter. That said, the final version of the big Conn sousaphones respected the .734" bore, but the tubing walls were much thinner. Nickel brass slides mostly started appearing on European instruments within the last few decades, but I do believe that thinner gauge sheet metal and tubing (which was common with European tuba-family instruments for a long time) was and has been related to overall weight, and that (I believe) was and perhaps sometimes is related to carrying for marching.
Everything adds up, particularly when someone's trying (whether for some practical reason or some misguided reason) to get a 24 or 25 lb. tuba down to only about 19 lbs. or so.
(I'm pretty sure that my 32-in tall 19-inch bell Holton B flat tuba - even - with those thin wall aftermarket King-fitting Meinlschmidt pistons - weighs a little bit more than my Miraphone model 98, and - if it does not - I'm pretty sure it would, were I to remove those silly and fairly heavy brass initials from the main tuning slide of the 98...The initials on the Holton are smaller and made of much thinner brass, for whatever that information is worth in this sidebar.)
The final sidebar here has to do with the flooding of the market with various levels of quality/various levels of usableness copies of a one-off tuba made in the 1930s for the Philadelphia Orchestra's
tubaist:
It's obvious enough to me (with very early pictures showing a very sharply etched fresh satin silver finish on the instrument discussed in this sidebar - actually appearing white in an often-seen picture) that this instrument suffered from having (sadly) had the holy crap buffed out of it at some later date. (Most people have no idea how much material must be sacrificed to rid an instrument of a satin finish - removing enough material to result in a glossy mirror-like finish.) In my personal opinion, that particular instrument has been devastated and (at least to me ~personally~ unless for resale) has no worth, as it's been structurally ruined (even though it might be mechanically sound. Attempting to match the current physical weight of that instrument with copies is a misguided pursuit, in my very opinionated (yet, I believe informed) opinion.