Brass is denser than steel, so it’s weight will lower the pitch for the same sizes and profiles. Brass also is considerably less stiff than steel which also lowers the pitch for equivalent sizing.
I have two Lachenal G/Ds, one brass reeded, and the other steel reeded. (both 20 buttons) Both from the 1800's. The steel reeded one has longer reeds, the brass reeded one has reeds more like a C/G, as far as length goes. I had a good discussion(online) with Geoffrey Crabb about this about five years ago, and he gave a good explanation of why brass reeds can be shorter, and steel ones need to be longer(in order to be lower pitched). And this is without solder or weights, etc...It all has to do with the different properties of brass versus steel, mechanics, physics, that sort of stuff. It was very informative, I wish I could remember exactly, maybe he'll chime in?
The more weight you have at the point of maximum travel you have ( reed tip ) the lower the pitch. The more stiffness you have at the point if maximum bending ( reed root ) the higher the pitch due to increased return force. Stiffness can be increased by using stiffer materials or by increasing the thickness in the profile. Weight can be increased by using denser materials or adding weight to the tip of the profile. Since the increase in weight at the tip has a more or less linear effect on pitch, low reeds can require a lot of extra weight to lower the pitch. The effect of adding thickness to the area of maximum bending has a much larger effect, ( as does subtracting it ) so flexible alloys can lower the pitch quite effectively. Other factors make a difference, like the maximum volume a reed can produce at a given size and swing. Reeds are a mass of trade-offs.