The majority of the Coalition forces at Waterloo weren't Prussian. The army composition of the Coalition forces is as follows:

Total: 118,000

- Prussians: 50,000 [1]

- British: 25,000

- King's German Legion: 6,000

- The Netherlands: 17,000

- Hanover: 11,000

- Brunswick: 6,000 [2]

Thus, the Prussians made up 42% of the Coalition forces gathered at Waterloo. The reason that Waterloo is considered by many to be a British victory and not a Prussian victory is twofold: one, because the lead commander of the British, Dutch and non-Prussian German forces was Arthur Wellesley, the Duke of Wellington, who was a British general (and later Prime Minister), although he was born in Ireland [3].

The second reason is because while the Battle of Waterloo begun at 11am, the first Prussian corps (of three) didn't arrive on the battlefield until 1:30pm, and the main force didn't arrive until 4:15pm. So while the battle itself would drag out until around 8:00pm [4], the majority of the Prussian soldiers were engaging the French army for less than four hours, the last hour of which was also mostly a rout as the French fled. Basically, the Anglo-allied forces fought the French for a full nine hours while most of the Prussians fought the French for just less than four.

To summarise: the Prussians didn't make up even half of the Coalition forces at Waterloo (only 42%), the overall commander of the Coalition forces at Waterloo was a British general, and the majority of Prussian forces engaged the French only for just less than half the time that the rest of the Coalition forces did.

References:

[1] Chesney, Charles C. (1874), Waterloo Lectures: A Study Of The Campaign Of 1815 (3rd edition), Longmans, Green, and Co.

[2] Barbero, Alessandro (2005), The Battle: A New History of Waterloo, Atlantic Books

[3] Gash, Norman (2011), Wellesley, Arthur, first duke of Wellington (1769-1852), Oxford Dictionary of National Biography (online edition)

[4] Roberts, Andrew (2014), Napoleon the Great, Penguin Books