Because silicon-based life would not thrive in an oxygen-rich atmosphere. Instead they would use other less abundant gases for respiration, given the low abundance of these gases they would be limited in their total energy intake - less reactants to preform respiration results in the organism staying very small in size as to sustain themselves. If they were to be larger, they would need to respire at greater extents - they would need more the reactant that is less abundant.
td;dr atmosphere limiting their potential growth.
