The Rambam says, "Anyone who eats an olive's volume of leavened bread on Passover beginning on the night of the 15th until the end of the 21st of Nissan intentionally is obligated to [a punishment of] kareit/cutting off."
why does one specifically get kareit for eating an olive's volume of leavened bread on Passover?
Seven days shall ye eat unleavened bread; howbeit the first day ye
shall put away leaven out of your houses; for whosoever eateth
leavened bread from the first day until the seventh day, that soul
shall be cut off from Israel. (Mechon Mamre translation)