Why does it say "Bitnecha Sa'achel"?
Radak: This is a Mashal. He should not vomit it out of his mouth, rather, feed it to his stomach.
Radak citing Targum Yonasan: Satiate your Nefesh.
Malbim: Do not eat it only with your mouth. Rather, it should descend to your stomach, and your innards will be full. Torah does not endure together with thoughts of worldly follies. Rather, your innards will be full of the Megilah - only Devar Hashem will be in you.
Why does it say "Es ha'Megilah"?
Radak: This is like Min ha'Megilah (your innards will be full from it).
How was it sweet in his mouth?
Rashi citing Targum Yonasan: I received it, and its words in my mouth were sweet like honey.
Radak: I learned it with desire and a good heart. The Lamed in "l'Masok" is extra, like "l'Amos Chamesh" (Divrei ha'Yamim II, 3:1).
Malbim: Aside from the good in Devar Hashem, it was also pleasant to his palate - "u'Mesukim mi'Devash v'Nofes Tzufim" (Tehilim 19:11).
