We show that the star-forming regions in high-redshift luminous andultraluminous infrared galaxies (LIRGs and ULIRGs) and submillimetergalaxies (SMGs) have similar physical scales to those in local normalstar-forming galaxies. To first order, their higher infrared (IR)luminosities result from higher luminosity surface density. We also finda good correlation between the IR luminosity and IR luminosity surfacedensity in starburst galaxies across over five orders of magnitude of IRluminosity from local normal galaxies to z ~ 2 SMGs. The intenselystar-forming regions of local ULIRGs are significantly smaller thanthose in their high-redshift counterparts and hence divergesignificantly from this correlation, indicating that the ULIRGs foundlocally are a different population from the high-redshift ULIRGs andSMGs. Based on this relationship, we suggest that luminosity surfacedensity should serve as a more accurate indicator for the IR emittingenvironment, and hence the observable properties, of star-forminggalaxies than their IR luminosity. We demonstrate this approach byshowing that ULIRGs at z ~ 1 and a lensed galaxy at z ~ 2.5 exhibitaromatic features agreeing with local LIRGs that are an order ofmagnitude less luminous, but have similar IR luminosity surface density.A consequence of this relationship is that the aromatic emissionstrength in star-forming galaxies will appear to increase at z>1 fora given IR luminosity compared to their local counterparts.