Most cellular phones rely on headset jacks that are sized at 3.5mm. The same size ports are also found on current models of laptop and desktop computers from all manufacturers.
Some older phones did have a smaller, 2.5mm jack, but as phone manufacturers diversified into making accessories most of these peripheral ports became standardized industry wide. This allowed for more manufacturers to create devices that were compatible with more phones while making it easier for consumers to purchase the product they needed without having to replace their phone in order to use it. Alternately, some phones and some headset manufacturers allow for USB and micro-USB cables to be used instead.