After melting the end of my MB15 I've upgraded to a MB25.
Question is do I need to up the flow rate on a bigger torch? Welds are looking duller and a bit more splattery than with my old one. (left flow at 15 lpm).
15L/min seems a bit high for an MB25. That's normal size and I tend to run 10L/min or less for mine. Though in general I think the bigger torches do tend to need a little more gas flow.
Technique-wise you might try a more vertical angle with the torch or holding it closer to the work, or pushing rather than pulling. All those ought to reduce the gas needed.
Though dull rather than rusty.... I'll get out while the going is good and admit I have no idea.