I can't access your CSS file, so I can't reply definitively, but the word "Plugins" is wrapped to the next line. So check your CSS for <li> settings that might cause a big gap below a line.
Also, there is an un-styled <span> around the word "Plugins" which is apparently doing nothing. You might try removing that to see if it makes any difference.
As an aside, I would suggest that this site needs a lot of work before it would be suitable for viewing on a mobile. The attempt at making it responsive is poor and of little benefit as the main nav bar seems to not be responsive, and is too wide to fit on many screens.