I dunno what the Whizzer capacities are but the reasons they look better is the lack of that huge split seam and I'm sure they got a deeper tunnel so they sit over the frame instead of on the frame. It's those 2 subtle differences that can make a big difference in how they look. I think somebody in here has taken a stock peanut tank, shaved off that seam and re welded it, then modified the tunneland it ended up looking a lot better.
Some of my ideas for better looking tanks were to use a 70's or early 80's minibike tank or a moped tank, then I saw the price of the moped tanks like the Puch tanks which can cost nearly $200, then some of the minibike tanks like the 1980 and older Suzuki JR50 tanks can be had for just under $100, and the post 1980 plastic tanks can be had for around $50 to $75 which also look decent on a bicycle since they're long, slender, and fit over the frame, but can hold a gallon of fuel. The Honda xr50 type tanks also look good but fitment takes some bracket fab work and they're designed for the seat to overlap so these don't look as good by themself, but do look really good if the seat can also be incorporated into the build.