The interdependency of microsoil fabric-related properties and swell/shrink behavior of expansive soils needs to be identified to achieve a more thorough and accurate prediction of heave of expansive soils. In a recent research study, two microsize-related properties of a soil—pore size distribution and specific surface area—were identified and examined in an effort to address their synergistic effects on swelling behavior of soils. To study this aspect in detail, eight natural expansive clayey soils from known expansive soil regions were sampled and studied by using two test methods: conventional one-dimensional vertical swell tests and novel three-dimensional volumetric swell strain tests. Microinternal structural details, including pore void distribution, were obtained from mercury intrusion porosimetry (MIP) studies on compacted soil specimens. Specific surface area (SSA) details of the same soils were determined using the chemical ethylene glycol monoethyl ether (EGME) method. Attempts were made to predict macroswell properties, using both pore and surface area properties. Modeling analyses and results showed that the swelling behavior of the clays was dependent on a newly introduced parameter that accounted for both clay mineralogy constituents and internal pore distribution of the soils. The new swell property models showed accurate predictions of measured swell strains of the present soils, depicting the importance of including mineralogy and pore fabric details in a given soil.