I think that the basic principle for growing mixes is being glossed over in this discussion; that growing mediums consist of a THREE way ratio of AIR : WATER : SUBSTRATE Increasing the particle size will increase the proportion of air in the mix, provided that the larger particles are a high enough proportion of the ingredients so that they touch each other in order to have air gaps between them, - the threshold for this to work is about 1/3. To use moisture retaining particles will make a damper mix, (I use leaf mould for this job - it helps stop sudden dryness which causes premature dormancy in many corms). Coarse leafmould, large particles of perlite, larger pieces of composted bark, (?pumice)... will all raise the quantity of air and water in the mix and reduce the proportion of substrate (and reduce the weight). Small particles of moisture retaining ingredients will increase the water content, reduce the air content and increase the water even more as well as the amount of substrate, and therefore the weight will go up. Small particles of non water retaining ingredients such as fine sand, clay, loam, rock dust.... will increase the water (due to capillary action) and substrate content, make a denser and heavier mix, and reduce the air content. Larger particles of non water retaining ingredients will make for a mix low in water, high in air, low in substrate and low or high weight depending on what the particles are made of (- usually gravel which is heavy). considering this then take the ingredients you have and mix for the air : water : substrate proportions which are desired, remembering the other properties of those ingredients such as weight, nutrient retention, PH... and your personal growing conditions. Peter (UK)