A mill is configured to have large debris passages disposed among a series of radially extending blades. The mill center is adapted to accept a retention nut that supports a tool that secures the downhole tool being milled out, such as a packer. Reverse flow takes cuttings into the large open area between the blades to pass up in an annular space around a support for the retention nut. The passage then opens up to a maximum dimension leaving only the tubular wall needed for structural strength to support the mill and conduct cuttings into a debris removal tool.