The phrase "are a tiny portion" is correct and usable in written English.
It can be used to describe a small part of a larger whole in various contexts, such as statistics, descriptions, or comparisons.
Example: "The results of the survey indicate that those who prefer option A are a tiny portion of the total respondents."
Alternatives: "represent a small fraction" or "constitute a minor part".
The Concordes are a tiny portion of the fleets of British Airways and Air France but showpieces nonetheless.
That's true if you're paying attention only to the celeb and politician videos, which are a tiny portion of the total.
The department's report notes, however, that those numbers are a tiny portion of the millions of travelers who legally enter the country each year.
Babies found dead or discarded in unsafe places are a tiny portion of those abandoned; most are left safely in hospitals.
Buyers from mainland China are a tiny portion of purchasers of high-end real estate in London, accounting for 5 percent of all purchases by foreigners of London properties valued from £500,000 to £1 million this year.
Moldova's soldiers are a tiny portion of the 25,000 allied forces now working alongside 138,000 American troops in Iraq, whose new government will regain sovereignty on Wednesday.
That is a tiny portion of the 5.2 million Americans estimated by the State Department to be living abroad.
That's a tiny portion of all merchants accepting credit cards, so it still has a lot of work to do.
The $49 million is a tiny portion of AOL's total revenue, and there is no clear evidence that AOL violated accounting rules.
The revenue, rebated to citizens, could allow each person to buy enough water to meet domestic needs, as domestic use is a tiny portion of total water use.
Financial Products, but that it didn't matter after a while, because "by the end it was a tiny portion of my earnings" — meaning his bonus, which grew to millions a year, represented most of his pay.
