Maybe if they had a real big ship they'd have a larger crew but honestly you cannot just add characters to the main cast willy-nilly when it's shounen you're talking about.
But if they had a huge cast they could have just added all of the halfway decent people to the crew after their asses got handed to them.
Speaking of all the halfway decent people...
I have a sneaking suspicion that the final boss of the main series will be the Gorosei who have obtained/always had an ancient weapon and are also probably pretty spry fighters for old geezers. Either that or it will be the Gorosei AND something unexpected like the Rebellion headed by Dragon as well. Either way, I bet that in the end, just about all of the defeated villains who weren't totally malicious douchebags and every single former ally will show up to assist the Strawhat Crew in overthrowing all the baddies and reestablishing a secure world order.
Expect to see Water Seven transformed into a ship that rivals an ancient weapon.
Expect to see a bunch of defeated pirate captains rallying under the king.
Expect to see former marines fight for justice.
Expect to see all the people in the omake inside cover pages show up for their own reasons.