Ok, thanks for the link but we are not sure if we understand what you mean. Do you mean sticky header? Because if that's what you mean, sticky header is not so big as the regular header and that's why logo is shrinking. This is the most proper behave as nobody want huge sticky header that overlap the content which is the most important part of page while scrolling. The only thing you can do is increase this logo a little bit to 55px and this can be done with the following css:
Comments
#Top_bar.is-sticky #logo img:not(.svg) { max-height: 55px !important; }