You really do not need much mix around a post in either a well dug hole.
The advantage with mixed concrete is that you get far better contact with everything (native soil and post) and maximum density with fewer voids.
After all, the native soil is far weaker than either good concrete or even a wetted dry dumped mix with voids. A dry dumped mix will never be as strong as wet mix that is properly proportioned.
Artistic Bob - In the Dallas area, a bagging company could not afford pumice or even a manufactured lightweight aggregate in a concrete mix. The best pumice is imported from Greece and is costly, but makes good concrete. Lightweight aggregate is cheaper because of energy costs and freight and it may be stronger than pumice if you really need high strength. Local heavyweight/normal are always cheaper because of local availability, a reliable supply. Most local concrete baggers use local aggregate (within 150 miles)Sakrete and all the others do not ship aggregate far because the product is price-oriented. I have seen bagging plants that bagged several brands and just switched bags when they made different brands.