Why specifically a fig on shabbos?

Question

Why is a dried fig specifically the quantity one must carry to be liable on Shabbos? I would have thought the size of something functional - an olive, egg, or even a barley corn - would be the prohibited measurement.

Rambam Shabbos (18:1) - הַמּוֹצִיא דָּבָר מֵרְשׁוּת הַיָּחִיד לִרְשׁוּת הרבִּים אוֹ מֵרְשׁוּת הָרַבִּים לִרְשׁוּת הַיָּחִיד אֵינוֹ חַיָּב עַד שֶׁיּוֹצִיא מִמֶּנּוּ שִׁעוּר שֶׁמּוֹעִיל כְּלוּם. וְאֵלּוּ הֵן שִׁעוּרֵי הַהוֹצָאָה. הַמּוֹצִיא אָכְלֵי אָדָם כִּגְרוֹגֶרֶת. וּמִצְטָרְפִין זֶה עִם זֶה. וְהוּא שֶׁיִּהְיֶה כִּגְרוֹגֶרֶת מִן הָאֹכֶל עַצְמוֹ חוּץ מִן הַקְּלִפִּים וְהַגַּרְעִינִין וְהָעֵקְצִין וְהַסֻּבִּין וְהַמֻּרְסָן: One who transports something from a private domain to a public domain or from a public domain to a private domain is only liable when he transports a [requisite] amount that is useful for something. And these are the [requisite] amounts for transporting: [Regarding] one who transports human food, it is like a dried fig-bulk. And [different foods] combine with one another. And that is when there is a dried fig-bulk from the food itself, besides the peels, the pits, the stems, the bran and the coarse bran.

Answer 1

The חידושי הריטב"א (succah 6b) explains that the Rabbis (Chachomim) assert that a kazayis is the minimum size that is fitting for eating so that became the standard for most eating related measurements (Yom Kippur being an exception) but it's still too small of a size to be significant with regard to carrying on shabbat whereas a fig גרוגרת is the minimum size of significance that one would be particular מקפיד to carry outside and anything less would be like a speck of dust.

Halachah lemaasah, one may not carry a speck of dust if one vies it with some significance or value and it's not just a speck of dust like it would be for the res of us.

Answer 2

It's a הלכה למשה מסיני brought in sukkah 6a.

Sefaria's version:

תאנה כגרוגרת להוצאת שבת

Fig alludes to the measure of a dried fig-bulk with regard to the halakhot of carrying out on Shabbat. One is liable for carrying food fit for human consumption on Shabbat, provided that he carries a dried fig-bulk of that food

.......

אלמא דאורייתא נינהו ותסברא שיעורין מי כתיבי אלא הלכתא נינהו וקרא אסמכתא בעלמא הוא

Apparently, all these halakhic measurements are derived from this verse in the Torah and are not halakhot transmitted to Moses from Sinai. The Gemara refutes this argument: And how can you understand it in that manner that all these measures are explicitly written in the Torah with regard to each of the halakhot mentioned above? Rather, they are halakhot that were transmitted to Moses from Sinai, and the verse cited is mere support for these halakhot, not a source.