ok, you need also to work with same "I" and "R" in both cases.
For your case, making a dynamic analysis by combining modal SRSS, the values on spectrum corresponding to second, third, etc modes may be on the linear ascending part of spectrum (usual such part of spectrum is constructed between ag and 2.5ag). As an extreme improbable case of "minimum" in dynamic calculation, let's say they correspond (all) to "ag" on spectrum and their effective mass participation is 50%. In this case a SRSS with the first mode 50% subject to 2.5ag is something as 1.34ag vs 100% mass subject to 2.5ag in "equivalent static". Of course such "demonstration" is ridiculous in terms of calculation, but my conclusion is that I would be suspicious if your wording " big in static analysis and so low in dynamic analysis" means something greater than a ratio of 2...2.5 to 1.