Since the Macintosch conventions of 1984 (I-312) it's sort of standard practice - apart from the 'X' or 'V' to appear when selected - how checkboxes and radio buttons are shaped. Square shapes for checkboxes and circular shapes for (mutually exclusive) radio buttons.
This - by the way - doesn't mean we should stick to this the rest of our lives, and there are already some new forms of mutually exclusive radio groups since the era of mobile devices (without the circular shape), but I'm wondering where the use of roundly shaped checkboxes used by Google inbox and the Microsoft outlook app come from. Any ideas?