I was looking at the screenshot on the linked page, and I failed to see where you think the megapixels were wasted? The channel list? Or the alignment of username/message?
I know that many modern apps follow this pattern and it may have to do with making things responsive and work well across many different devices and platforms, but I simply cannot be happy with low information density.
YMMV. It's good that there are so many different clients to chose from. IRC still rocks.
That being said, The Lounge supports themes, and it's trivial to reduce spacing to fit your needs. There's even a box in settings where you can put custom css directly in.
I mean, HexChat use roughly the same space (the username part is configurable), so this is hardly a problem of 'modern UI'. (XChat, which HexChat fork from, was initially released in 1999)
Not the guy you asked, but isn't that the default? I've always done '/names' to see who was there and '/list' to see what channels were available.
That being said, I think the whitespace between the lines was the complaint (IIRC the default is a line size of 1.4 and 3px padding)
With a little CSS I eliminated the WS, and here's a screenshot with the two sidebars hidden: