Vincent's NT Word Studies
31. When it is sown (otan sparh). This phrase is repeated in verse 32. Here the emphasis is on otan, when. It is small at the time when it is sown. In verse 32 the emphasis is on sparh, it is sown. It begins to grow great from the time when it is sown.That are upon the earth. A little detail peculiar to Mark.
Robertson's NT Word Studies
4:31 {When it is sown} (hotan sparei). Second aorist passive subjunctive of speirw. Alone in Mark and repeated in verse #32. {Less than all the seeds} (mikroteron pantwn twn spermatwn). Comparative adjective with the ablative case after it. Hyperbole, of course, but clearly meaning that from a very small seed a large plant grows, the gradual pervasive expansive power of the kingdom of God.