“That sounds easier to stick to.”
Stick to something means to keep following a plan, rule, or decision. It is common when discussing diets and habits. Frank thinks this approach sounds easier to maintain.
B1-42 / CEFR B1 / EPISODE 42
Revising a plan so it is easier to follow
SceneTina is at Frank's apartment, telling him about her latest diet.

WORDS & CONTEXT
Stick to something means to keep following a plan, rule, or decision. It is common when discussing diets and habits. Frank thinks this approach sounds easier to maintain.
This explains the punch line. Frank suggested eating only two slices. Tina orders a large pizza cut in half, technically producing two slices while still letting her eat half a pizza. The joke is her self-serving interpretation of the agreement.