In the Ch.18, book of QFT by Mark Srednicki (P118), it says the diagram have the same mass dimension with tree diagram with the same external lines, because both of them contribute to the same scattering amplitude. But I am confused of the reason of this argument.

In the Ch.18, book of QFT by Mark Srednicki (p.118), it says the diagram have the same mass dimension with tree diagram with the same external lines, because both of them contribute to the same scattering amplitude. But I am confused of the reason of this argument.