A.T. Robertson Commentary Mark 4:31

"It is like a grain of mustard seed, which, when it is sown upon the earth, though it be less than all the seeds that are upon the earth," — Mark 4:31 (ASV)

When it is sown (οταν σπαρη). Second aorist passive subjunctive of σπειρω. Alone in Mark and repeated in verse 32.

Less than all the seeds (μικροτερον παντων των σπερματων). 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.