Small pieces of olivine have greater surface area to volume ratios and weather faster than larger pieces. Many proposals for olivine weathering are priced high and inefficient due to the energy use required to grind minerals down to small particles.
Project Vesta gets around this issue by only milling rocks down to pebble size, and allowing the grain-on-grain collisions and the abrasion introduced from the mechanical activaiton of olivine in the surf.
Life cycle assessments show that by minimizng the transport distance to 300 km (186 mi) and grind size to milimeters, we can keep the process 94%-95% efficient. Meaning that for each 1 tonne of olivine weathered, we are only generating .05-.06 tonnes of CO2. In other words, means our process can remove 20 times more CO2 than is generated in the process.