Problem with logo size (scrolling) with split menu

Hi there,
unfortunately I got a little problem with the logo size in my layout. Maybe it would be possible to help me with that?
I already had a look into other posts, but couldn´t find anything really suitable for my case.

My website is setup with a split menu and a logo which overlaps onto the slider beneath. That works fine.
Now I would like the logo – while scrolling down the page – to shrink and become so small to fit into the header completely.

Your help would be very much apprechiated :)