Some maintain that one is obligated to feast and rejoice on the 14th of Adar I. This is not our custom. Nevertheless, one should dine somewhat more than usual, in order to fulfill his obligation according to those who are stringent. “And the glad of heart, feast constantly (Mishlei 15:15)”. – Rem”a, OC 697

יש אומרים שחייב להרבות במשתה ושמחה בי”ד שבאדר ראשון ואין נוהגין כן
מכל מקום ירבה קצת בסעודה כדי לצאת ידי המחמירים וטוב לב משתה תמיד – רמ”א, ס”ס תרצ”ז

The Ta”z comments that some Rishonim actually celebrated Purim Katan with special Seudos. Accordingly, he explains the Rem”a’s concluding words, “the glad of heart, feast constantly”, as an indication that one should celebrate on Purim Katan, more than just “dining somewhat more than usual”.

Why then does the Rem”a merely hint to this by matter-of-factly quoting this verse from Mishlei, instead of saying so explicitly?

This indirectness reflects the unique nature of the joy of Purim Katan.

Each of the holidays, Purim included, has unique obligations in addition to the Mitzvah or custom to be joyous. Consequently, the celebration of the holiday is a combination of the yoke of responsibility to fulfill the obligations of the day mixed with the joy of Yom Tov. The purest joy is only on Purim Katan, in which there are no other unique Mitzvos.

Chassidus explains that the celebrations which the Jewish people instituted on our own initiative, either as a Rabbinic obligation or as a Minhag – custom, reflect a more profound joy than a joy in which the Torah obligates us. Rather than being imposed upon us as a command, these self-initiated celebrations indicate our personal joy of being G-d’s nation and associating with Him through the Mitzvos.

It is therefore with specific intention that the Rem”a does not directly record the practice of fully celebrating on Purim Katan. This pure joy of being a Jew is so profound that it must come entirely from within, and must not be “mitigated” by being imposed or instituted as standard practice.