It just won't charge as fast as with the stock charger (phonearena.com)
At any rate, if the LED remains on when the display has timed out, and the device should be in standby, the recharge rate is incredibly slow when using the stock charger. (carrypad.com)
The bigger Google phone likely conforms to the 18W charging through the stock charger, meaning that its peak charging speed will ordinarily be higher than the Pixel. (xda-developers.com)