A lot of UX guidelines revolve around finger tip size and these guidelines vary in recommended size and spacing. Finger tip sizes should not be the sole factor in determining what minimum size controls and spacing should be.
Consider the actual 'contact patch' made by the finger/thumb when making contact with a touchscreen. You also need to consider errors in judgment made by the user in thinking they have visually centred their touch on the target but in fact the actual contact made with the screen might be slightly off to one side.
Due to ways that different devices are held and the difference in distance from the user (compare mobile phone to a tablet for example, a phone might be held closer), minimum touch sizes and spacing can vary too. You could arguably get away with smaller minimums on a phone because they are held closer to the viewer and mistakes less likely to be made. For a larger device like a tablet held further away, you may need to increase your minimum sizes to cater for higher chance of mistakes.
There is a great article talking about this and other factors on UXmatters (http://www.uxmatters.com/mt/archives/2013/03/common-misconceptions-about-touch.php).
Snippets from the article:
...only part of the finger or thumb makes contact with the
screen...the contact patch varies by pressure and angle
Physical sizes matter, so all good guidelines are in millimeters,
inches, typographers’ points, or other real-world scales.
there’s no need to increase the size of the visible target. Instead,
you can simply increase the dimensions of the clickable area around a
link or button
Design for both visual and touch target areas. Consider users expectations.