As to the question in your title, consider country A with education spending of $1 million and a GDP of $5 million (a small country!), and county B that spends $1 billion on education, but has a GDP of $900 billion. (using U.S. usage, so 1 billion is 109).
Then although country B spends 900,000 (9 × 105) times more than country A, as a percentage (proportion) of its GDP, country B spends barely 0.11% (1/900), while country A spends a whopping 20% (1/5). So yes, that can happen easily, just as paying the same price for any good or service constitutes a larger proportional hit for a person of little wealth than for a person with great wealth.
A more natural way to convey your meaning is [Of the countries considered,] Thailand spends the largest share of its GDP on education.