it's because you have the apps, text and other items in scale layout in settings/display over the recommended 100%, reducing it back to 100% recommended will get the album artwork back to normal but all the text, apps and other items will reduce back to normal. this is how I managed to get bigger text on my desktop PC, it doesn't make the apps bigger but it works for me OK.
are you using Windows 10 and the latest Microsoft Edge browser, in Windows settings ease of access "display" make text bigger drag the slider (hold mouse on the slider to show you the size of the text) to the size text you want then click on apply, in Microsoft Edge browser, click on the 3 dots (. . . ) in the top right corner,go to settings then go to appearance then on that page you will see zoom, go across to the right side of the page from zoom you will see the number will be 100% click on the down arrow beside the 100% and it will give you more % to increase/decrease from 25% to 400%, I'm using both Windows 10 and Microsoft Edge (mine is set at 110%) and doing this the albums on iTunes are not grey squares, but the downside of this, is that the text in iTunes doesn't increase and the apps on the computer don't increase in size