Bugger,
Back when 3/4" big game scopes were common, glass was pretty mediocre and uncoated. As a result, the lenses couldn't "bend" light nearly as well as present glass and coatings, so 3/4" tubes didn't constrict the light-path much. Which is why older scopes were longer for the diameter.
You can tell if the tube is constricting the transmitted light by measuring the exit pupil. If it's smaller than the objective lens diameter divided by the magnification, then the tube is having an effect.
