Anode efficiency is dependent on a number of factors. Surface area of the spheroids is critical, hence smaller diameter is, as you said, a key requirement. Typically, battery producers use spheroids of 10 to 25 micron diameter, which are conveniently produced from flakes of 100 micron diameter or less. Anything larger just has to have the edges milled off, to get it down to the required size.
The counterbalancing factor is that most of the traditional suppliers, from China, had great difficulty in producing 99.9% quality from fine flake, whereas large-flake was of higher purity, and less costly to get to the required grade. That has now changed. SYR, for example, with ore of 18-20% graphite, and a demonstrated ability to produce 99.9% with flotation plus a low-temperature wash, will be in a position to provide large quantities of the preferred product, at relatively low cost. Whether this is sold directly, or used as input to their own spherical graphite plant, or both, is yet to be decided.
Energiser, with their poor ore grade and lack of supporting infrastructure, will doubtless be able to produce spherical graphite from their product, but their margins are likely to be significantly thinner.
