Waterborne and water-reduced coatings are increasing in relevance in many sectors as an alternative to solventborne coatings. In this work, the internal structure of waterborne polymers as a function of colloid particle size is unveiled and directly related to macroscopic water absorption. To this aim, a set of acrylic waterborne films was prepared from dispersions of different colloidal particle sizes (100, 150, and 200 nm) with the same surfactant coverage. Macroscopic water absorption and water affinity were studied by Dynamic vapor sorption (DVS) and immersion tests. Small-Angle Neutron Scattering (SANS) was used to study deuterated water diffusion with time. This revealed the presence of remnant hydrophilic colloid-colloid interphases in all films, independently of the forming colloidal size and annealing conditions. Moreover, fitting of SANS data revealed that water transport in these films happens through surfactant-rich colloid-colloid interphases or through 10 nm-wide hydrophilic paths rich in surfactant aggregates (in the range of 4 nm) when these are present. The presence of the hydrophilic paths explains the higher water uptake measured in waterborne films made from 100 nm colloids, a process so far not previously reported. This study highlights how water diffusion in waterborne films may be engineered through fine control of particle size and film formation conditions.