Viewing post in The Main image /browse window is so BIG
if it looks normal on your mobile that tells you that it is not the page that is different. it is your browser.
the style sheet on itch displays the page differently depending on available space. did you check your zoom factor for the page?
try resizing the window, like halfing its size horizontally. and try increasing your zoom factor and decreasing it.
the bigger banner is like a house roof. it takes all the place over the rows and colums of games under it. so it stays the same, if you have 3 or 4 or 5 columns