It states (Esther 4:17) ויעבור מרדכי, and the Gemara says that this means Mordechai crossed over a body of water to go to the Yidden who lived on the other side of Shushan, to tell them to daven for Esther. Manos HaLevi (written by Reb Shlomo Alkabetz zt'l, author of Lecha Dodi) explains, "Even a minor deed, like crossing over water, is considered a great deed when done for a mitzvah. The water wasn't a large river; it was more like a puddle... Nevertheless, the Megillah tells us that Mordechai crossed the water as though it was a great accomplishment. This is because everything is accounted for."
When a person performs a mitzvah, even the slightest deeds are worthy of being written in the megillah for all generations to read and see.
Chazal (Gittin 57:) state, "The grandsons of Haman taught Torah in Bnei Brak." Why did Haman HaRasha merit this? Shem MiShmuel (Purim 5680) writes it is because Haman had one moment of emunah. The Midrash (Esther 10:5) states, "When Haman led Mordechai on the king's horse through the city's streets, what was Haman the rasha saying? (Tehillim 30:7-8) ואני אמרתי בשלוי בל אמוט לעולם... הסתרת פניך הייתי נבהל, 'I said in my tranquility, 'I will never falter.' ... You hid Your countenance, and I became frightened.'" He had a moment of emunah, and for that, he was rewarded with descendants who taught Torah.
No one is worse than Haman, yet see how much he gained from one good thought! So, even when you learn just one chapter of mishnayos or one halachah, etc., these small deeds are extremely precious to Hashem, and the reward will be enormous – greater than we can imagine.
It states (Esther 3:2) ומרדכי לא יכרע ולא ישתחוה, "Mordecai would neither kneel nor bow down." It is written in the future tense. Sfas Emes zt'l says that alluded here is a lesson for every Yid. Even if in the past, there were times he kneeled and yielded to the yetzer hara, he should tell himself that he won't do it again. Or, we can explain it this way: Even if he kneeled (יכרע), he wouldn't bow down (לא ישתחוה). He will stop the influence of the yetzer hara in its tracks and not allow himself to descend to lower levels.
Shem MiShmuel writes, "This is a lesson for every person: Even when he is at a low level, r'l, he shouldn't consider any good thought or good word that he says to be small. Let him grasp whatever good moments he can, and this will be a great benefit for him. The mouth cannot express how great these deeds are."
The Gemara (Megillah 16.) states that when Haman came to Mordechai (to lead him through the streets of Shushan, as Mordechai rode on Achashveirosh's horse), he found Mordechai teaching the laws of kemitzah to his students. Kemitzah is to bring a handful of a meal-offering (made of just flour and oil) on the mizbeiach. Haman thought that small deeds are unimportant; only great deeds have value. This was demonstrated by the enormous gift of ten thousand kikar silver coins he offered Achashveirosh. In his eyes, only tremendous and impressive matters have value. Mordechai, on the other hand, taught his students that even a small handful (a kometz) of a minchah is a korban in Hashem's eyes. Every small deed is very precious in Hashem's eyes. In the end, Haman admitted that Mordechai was correct and said, "Your handful of flour pushed away my ten thousand kikars of silver."
One of the benefits of the "small deeds" is that one good deed leads to another, as Chazal say מצוה גוררת מצוה.
Chazal (Shabbos 105:) say, "This is the way of the yetzer hara: Today he tells you to do this [aveirah], tomorrow he tells you to do that [aveirah], until he tells you to worship avodah zarah – and he worships it!" The yetzer hara is happy with minor sins because he cleverly knows that one deed leads to the next. We should learn from our enemy and consider every good deed significant. It is great in Hashem's eyes, and furthermore, one small deed will lead to another good deed, and gradually, we will grow in avodas Hashem.
