"Be as careful [in Hebrew, "zahir"] in [the performance of a seemingly] minor mitzvah as with a major one." (Avot, 2:1)

Careful

The word "zahir" (translated here as "be careful") is related to the word "zohar", meaning "light", and hints to the soul. Thus, with this meaning, the above statement teaches us that the soul will be illuminated by a "minor" mitzvah just as it is illuminated by a "major" mitzvah because, "G‑d desires the heart". (Baal Shem Tov, Tzva'at HaRivash 1:17)

In all the commandments

The 248 positive commandments are called the "limbs of the King". Just as physical limbs are made up of flesh, bone and sinew and act as vessels containing the light and life of the soul, so too the mitzvot are vessels for the revelation of G‑d's Will.

Each limb of the body has a particular function, and its own unique nature and characteristics. And all of the limbs together, since they are interconnected and interdependent, form a complete and whole person. The following examples illustrate these ideas: If one of a person's limbs or organs, whether internal or external, is damaged, the entire body does not function correctly. The same principle applies to healing - by letting blood from a healthy limb, another section of the body which is ill can be healed. Similarly, when a person occupies himself with work which he does with his hands, he also occupies other powers of his soul, such as his intellect, his power of sight, his hearing etc. If one limb is missing or is damaged, it affects the entire person who becomes crippled or maimed. Each mitzvah has its unique intention and draws down divine light in its own particular way…

So too with mitzvot which are "limbs of the King". Each mitzvah has its unique intention and draws down divine light in its own particular way, according to the distinctive nature of that mitzvah. In addition, all of the mitzvot are interconnected and, by fulfilling each mitzvah, a person connects to the supernal light in all of the mitzvot together. And, just as a person will expend all his energy in avoiding becoming maimed in one of his limbs, so must he do in a spiritual sense. He must make every effort not to be maimed in one of his spiritual "limbs" - even in one mitzvah. Therefore, our Sages tell us, be careful in a "minor" mitzvah just as in a "major" one.[Rabbi Dov Ber Shnueri, Torat Chaim, Ex. p. 493]

Connecting

The different levels which we find in the mitzvot ("chukim" - those mitzvot which are super-rational and cannot be understood; "eidut" - mitzvot which one would not necessarily have arrived at by logical inference but which can nevertheless be understood; "mishpatim" - mitzvot which can be understood rationally; minor, major) stem from the intellectual aspect of each mitzvah. However, regarding the essence of each mitzvah - all stem from G‑d's Will, and there is no difference between them.

The main aspect of all mitzvot is that by way of them a person becomes connected and bound with the Holy One, blessed is He. We see this illustrated in the fact that the word "mitzvah" is related to the word "tzvata" meaning a bond or knot. And this is equal in all mitzvot. When a Jew fails, G‑d forbid, to perform even a minor Rabbinical amendment, this also transgresses G‑d's Will and therefore affects his entire connection to Divinity. For this reason the Mishna warns, "Be as careful in a minor mitzvah as with a major one" - you must take the same care in performing all of the mitzvot. (The Lubavitcher Rebbe, Likutei Sichos vol. 13, p.69)