You need to open your SVG icons file with some editor like Sublime Text or default system text editor and add width=”31″ height=”40″ attributes to your <svg> tag like it is shown: http://prntscr.com/m4n0qw
Then you will need to re-upload your icons in WordPress Dashboard.
Thanks for your answer
As a solution, I have finally uploaded the icons to https://icomoon.io/ and downloading the files it generates has worked correctly.