Can you replace 12v 33Ah battery with a 12v 7Ah battery?

The Ah (amp hour) rating of a battery is a measure of it's capacity. A 33Ah battery, when fully charged, will deliver 33 amps for one hour, or 11 amps for 3 hours, etc.
A 7 Ah battery will only last about 20% as long as a 33Ah battery.