The classical 7805 had a fold back characteristic. The higher the voltage drop between input and output the lower the current limit to stay inside the safe operation area of the transistors. In addition the current limit depends on temperature. The better the cooling (the lower the temperature of the chip) the more current you can get.
Today 7805 is manufactured by a lot of companies and the implementation may differ from manufacturer to manufacturer.
If you are willing to add a PNP external power transistor you can boost it. (but you loose short circuit protection). Run the input of the 7805 through a 10 Ohm resistor. Connect the emitter of the PNP to the high side of the resistor and the base to the 7805 input on the other side of the resistor. Connect the collector to the output of the 7805.
Probably you will have to change the output capacitor to achieve stability again.
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