The nanoporous metal-organic framework material Cu-3(1,3,5-benzenetricarboxylate)(2)(H2O)(3)center dot{guest} exhibits anomalous compression under applied pressure that is associated with the hyper-filling of the pore network. This behavior involves a dramatic transition between a "hard" regime (bulk modulus, K-hard approximate to 118 GPa), where the pressure-transmitting fluid penetrates the framework cavities, and a "soft" regime (K-soft approximate to 30 GPa), where the guest-framework system compresses concertedly. Not only is the duality in compressibility triggered by the availability of potential guests but the size/penetrability of the guest molecules determines the pressure at which the hard-soft transition occurs. Specifically, the observed compression behavior depends on the size of the pressure-transmitting fluid molecules, the sample particle size (i.e., the extent of the pore network), and the rate at which the pressure is increased. The unprecedented pressure-induced phenomena documented here, illustrates the exotic high-pressure behaviors possible in this versatile class of advanced functional materials with broad implications for their structure-function relationships and accordingly their practical application.