The authors have used various sized particles of phosphors and have found that within the range of sizes used, the smaller the size the greater is the intensity of fluorescence. The graph bears out this relation. The phenomenon of fluorescence seems to be a surface phenomenon.
M. S. OLDHAM and WILLIAM KUNERTH, "The Effect of the Size of the Particle on the Intensity of Fluorescence of a Phosphor," J. Opt. Soc. Am. 31, 102-104 (1941)