Most of the lines have the same structure, with "a lot" occurring at the end ("Eat a lot / Sleep a lot... Run a lot / Do a lot"), and this repetition (technically called "epistrophe") matches the great quantity that the phrase itself indicates. The song contains repetition on a broader scale, too, since the same verse recurs throughout, just with melodic differences.
In "Eat a lot," however, "a lot" could function as either a direct object ("eat an abundance") or an adverb ("eat frequently"), and this duality of possible meanings also hints at a large amount. Similarly, with "Sleep a lot," "a lot" could indicate duration ("sleep for a long time") or frequency ("sleep often").
