Our gemara on amud beis references the virtue of the righteous who say little but do much.  

This is most famously memorialized in Parashas Chaye Sarah where Avraham offers his guests a little water and bread, but actually provides them with a feast of delicacies (see Avos Derabbi Nosson 13:3).

My father, Z”L used to say to me, “Simcha, do not commit too much, as the Yetzer Hara will overhear and try to stop you.”

I do not know if my father once learned this from his rebbeim, or he just intuited it, but recently I saw a Chida (Penei David Chaye Sarah 6) that explains this along the lines of my father’s wise counsel.

He says that a Tzaddik verbalizes that he will do something, so he benefits from the engagement and creation of an idea. This also to a degree arouses the yetzer hara to stop him but also increases his reward for fighting it.  Yet, because he only committed to a small amount, Satan is mollified and does not bother to disrupt the person much.  Then, at the last minute, the person, so to speak, surprises Satan by doing a much greater deed.

Psychologically we can understand that when a person over commits, it can be a sign of Reaction Formation.  Reaction Formation is an ego defense whereby a person unconsciously loathes something, but to fight this unseemly idea, overrides those feelings and states the opposite.  Thus a person might magnanimously invite an enemy to his feast because deep down he is ashamed that he hates him. Or as Ephron did, tell Avraham insincerely (Bereishis 23:15):

אֲדֹנִ֣י שְׁמָעֵ֔נִי אֶרֶץ֩ אַרְבַּ֨ע מֵאֹ֧ת שֶֽׁקֶל־כֶּ֛סֶף בֵּינִ֥י וּבֵֽינְךָ֖ מַה־הִ֑וא וְאֶת־מֵתְךָ֖ קְבֹֽר׃ 

“My lord, do hear me! A piece of land worth four hundred shekels of silver—what is that between you and me? Go and bury your dead.”

Which, according to Chazal, was a “dog-whistle” to Avraham that this is exactly what he wanted to be paid (ibid 16 and Rashi Op. Cit, quoting Bava Metzi’a 87a):

וַיִּשְׁמַ֣ע אַבְרָהָם֮ אֶל־עֶפְרוֹן֒ וַיִּשְׁקֹ֤ל אַבְרָהָם֙ לְעֶפְרֹ֔ן אֶת־הַכֶּ֕סֶף אֲשֶׁ֥ר דִּבֶּ֖ר בְּאָזְנֵ֣י בְנֵי־חֵ֑ת אַרְבַּ֤ע מֵאוֹת֙ שֶׁ֣קֶל כֶּ֔סֶף עֹבֵ֖ר לַסֹּחֵֽר׃ 

Abraham heard Ephron’s terms. Abraham paid out to Ephron the money that he had named in the hearing of the Hittites—four hundred shekels of silver at the going merchants’ rate.

It is equally important and notable that the dictum isn’t, “Tzaddikim say nothing and do much” , but rather “Tzaddikim say little and do much.”  There is a value to verbalizing some commitment both internally and relationally.  That is, to help organize one’s thoughts and values, it is a human need to articulate and talk it out, and also the other person feels honored when you let them know your intentions, even if you plan to do much more.