Young Spock didn't go to Nero's ship with the goal of finding a ship. He intended to find the black hole device (which he had obviously witnessed in action) and steal it (or if that wasn't possible, probably destroy it).
SPOCK: Doctor, Mister Chekov is correct. I can confirm his telemetry. If Mister Sulu is able to maneuver us into position, I can beam aboard Nero's ship, steal back the black hole device, and if possible, bring back Captain Pike.
KIRK: I won't allow you to do that, Mister Spock.
SPOCK: Romulans and Vulcans share a common ancestry. Our cultural similarities will make it easier for me to access the ship's computer to locate the device. Also, my mother was Human, which makes Earth the only home I have left.
Once aboard the Narada (and after a short battle):
(Spock melds with an injured Romulan)
KIRK: Do you know where it is? The black hole device?
SPOCK: And Captain Pike.
He probably learned during that mind-meld that the device was incorporated into a small ship that was aboard the Narada. Since he's (half) Vulcan (and he's excited that he knows where Pike is), he hides his surprise well. The only hint that he expected a ship before he actually saw it is when he boards it for the first time:
SPOCK: I foresee a complication. The design of this ship is far more advanced than I've anticipated.
