This paper presents an investigation into the size reduction characteristics of particle beds under confined compression. The test data were chosen from a series of specifically designed compression tests and factors including particle materials, feed size and gradations were considered. Through extensions of previous appearance functions mostly applied in single particle breakage tests, we established descriptive functions for fragmentation degree indices using input energy and particle size, and developed relationships between characteristic indices to predict size reduction. Based on the established correlations, some discussions on fine products distribution and the applicability of self-similarity are also presented. Our test results, particularly the distribution of fine fragments, demonstrate the suitability of self-similarity in the product size distributions after sufficient compression and breakage. The present study is deemed as a supplement to the existing approaches for describing the size reduction response.