The polydispersity of primary particles (PP) in agglomerates is important in a number of applications, including quantum dots, nanocomposites and even for pigments. Accounting for the PP polydispersity during sintering, their number and size distribution are conserved once agglomerates are formed by a new two-dimensional sectional model for coagulation and sintering. By balancing the complete PP population as a function of agglomerate size, natural and numerical narrowing and broadening of the primary particle size distribution (PPSD) are demonstrated systematically. Natural narrowing of the PPSD in agglomerates arises by the faster sintering of smaller PP compared to larger ones. Numerical narrowing of the PPSD occurs when average PP diameters are employed when calculating agglomerate coagulations and sintering. Numerical broadening of the PPSD by numerical diffusion is caused by the fixed primary particle spacing, similar to aerosol growth by condensation. Agglomerate and primary particle dynamics are elucidated during titania formation by detailed two-dimensional (primary and agglomerate) size distributions showing that the PPSD can be much narrower than the well known self-preserving size distribution of agglomerates by Brownian coagulation, especially when hard-agglomerates are formed.