Some Mitzvot are written in the Torah very briefly or even hinted to, like "כבד את אביך", "לא תבשל גדי", "וקשרתם לאות על ידך" and many more. For example, the Torah gives no details on how the כיבוד אב should be performed - we learn it all from the Oral tradition.
Some Mitzvot include miles and miles of details in the TOrah itself, like Chometz on Pesach or returning a lost property (Dvorim 22):
"לא תראה את שור אחיך או את שיו נדחים והתעלמת מהם השב תשיבם לאחיך: ואם לא קרוב אחיך אליך ולא ידעתו ואספתו אל תוך ביתך והיה עמך עד דרש אחיך אתו והשבתו לו: וכן תעשה לחמרו וכן תעשה לשמלתו וכן תעשה לכל אבדת אחיך אשר תאבד ממנו ומצאתה לא תוכל להתעלם"
That's how the Mishna puts it in its own words (Chagigah 1 tnx Joel!):
"הֶתֵּר נְדָרִים פּוֹרְחִין בָּאֲוִיר, וְאֵין לָהֶם עַל מַה שֶּׁיִּסְמֹכוּ. הִלְכוֹת שַׁבָּת, חֲגִיגוֹת וְהַמְּעִילוֹת, הֲרֵי הֵם כַּהֲרָרִים הַתְּלוּיִין בְּשַׂעֲרָה, שֶׁהֵן מִקְרָא מֻעָט וַהֲלָכוֹת מְרֻבּוֹת. הַדִּינִין וְהָעֲבוֹדוֹת, הַטָּהֳרוֹת וְהַטֻּמְאוֹת וַעֲרָיוֹת, יֵשׁ לָהֶן עַל מַה שֶּׁיִּסְמֹכוּ. הֵן הֵן גּוּפֵי תּוֹרָה:"
Dissolving vows fly in the air, there is no basis for it. Laws concerning Shabbat and festival-offering and stealing from holy-designated things, these are like mountains hung from a hair: they have few verses and many laws. Judgment and service and purity and impurity and improper sexual relations, they have plenty to be based on. They themselves are the body of Torah.
Why is it so? Why are some details called the Written Torah and the others the Oral Law?
(please answer in general, not for those specific Mitzvos, like "if the Torah wasn't saying X we wouldn't know".)
Please note that my question deals specifically with the fine balance between the written and the oral law - where the written Torah stops and the oral law begins, unlike this question that deals only with the details of the stories in the Torah.