You must have astonishingly stringent qualifications if the largest empire in history doesn't qualify.
I don't know how to formulate it without sounding pedestrian, but it's not the size what matters to me.
I like to use a more holistic approach, years, cultural influence, art, cities, etc.
With no disrespect intended to Ferdinand, who was powerful enough in his own right, it's well known that Isabella was the real power in the relationship; Ferdinand was mostly along for the ride.
A bit off-topic, but although I'm quite familiar with that theory, most of the historical facts and documentation are against it. To the point that after Isabella's death, it was precisely Ferdinand who ruled Castile as regent (when he was allowed after Philip I of Castile died). And he always kept a complete independence of the Crown of Aragon's countries from the Crown of Castile; status that lasted until 1705-1716's decrees of New Plant.
That theory was spread widely during the Spanish Dictatorship of Francisco Franco as a way to justify Spanish nationalism (even nowadays the dynastic union is still used by some Spanish nationalist politicians to defend the birth of a unified modern nation, when nationalism is something of the 18th century-forward). You only have to check Spanish history books from the early 1930s and the ones produced during the dictatorship or even nowadays.
In case you are not aware, the dictator Franco and the Spanish fascist and only legal party "La Falange", used a lot the Catholic Kings symbols, specifically the yoke and the arrows .
https://en.wikipedia.org/wiki/Symbols_of_Francoism#Escutcheon
Thing is that, saying that Isabella from Castile was the one that had the control of the relationship, is a mean to justify the supremacy of the Castilian culture over the others (the Crown of Castile was totalitarian and the most used language was Spanish, against the pact-friendly Crown of Aragon where each country had its medieval parliament which whom each monarch had to negotiate with).