Always like this with RCA vs XLR. Technically, gain is the amount by which the input signal is multiplied. Gain is always fixed. Gain is fixed in amps, gain is fixed in preamps. You do not change gain other than by redesigning or modifying the amplifier stage within the component.
What you are seeing is not gain. It is simply different input voltages. XLR always because of he way they are wired result in greater input voltage than RCA.
@millercarbon Just so you know, this isn’t how its supposed to work. In a balanced system, the signal is not generated with respect to ground; ground is independent of the signal. So the RCA and XLR outputs should be the same voltage!
Think of a phono cartridge, which is a balanced source. Its output does not change if you run it balanced or single ended. The ground is the tonearm tube; its independent of the signal. Think of an output transformer, whose secondary can drive either single-ended (with one side at ground) or balanced (neither side at ground; ground is merely chassis with no other connection to the transformer). The output voltage is the same in either case.
When you see the gain difference as described, what is happening is that the inverting and non-inverting aspects of the balanced line are generated with respect to ground rather than each other. IOW, each is a single-ended output, with one out of phase with the other. This type of connection does not support the balanced line standard, since the ground connection is essential for its operation to complete the circuit.
So its not ’always like this’ as you say, its just like that if the equipment isn’t supporting the standard.