They are not all created equal or Successful Single Shots
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It seems strange to have a Wiki article devoted to the humble filter basket, but they are an often overlooked part of coffee preparation. The portafilter (or group handle) attaches to the group espresso machines (in our case a 58mm group or standard E61 bottom group), and carries a tamped puck of coffee grounds within its basket. It is usually made of brass in the prosumer machines, and has a plastic or wooden handle attached. The portafilter itself does not form a seal with the espresso machine's gasket, this is actually done by the basket. This seal/basket combination directs high-pressure (140 psi or 9 bar) hot water through the coffee puck.
Bottomless (or naked) portafilters limit the espresso's contact with metal and is a useful tool to analyze the quality of the coffee grind and the evenness of the extraction (shown with a double basket insert in picture).
This article focuses on the standard E61 58mm filter baskets. So let's have a look at a lineup of typical triple1, double and single baskets (as usual click to enlarge).
Now it will be immediately apparent that the main difference between the triple and the double basket is height (plus the bottom has no tapered shoulders) and a triple will not fit in a normal portafilter, although it may fit in your naked portafilter, depending on how wide the "cut" is at the bottom, some homemade ones may have a problem.
The singles though show a marked variation in size, and broadly divide into two types I call "large" and "small" singles. let's look a little more closely at these, compared to a standard double basket:
I like this particular "large" single, as you can see in comparison with the small single below, it holds a little more coffee (before it touches the shower screen) and gives a much superior extraction with less risk of "channelling" or other problems such as early blonding. the taste is also slightly better, I think, partly due to the slightly coarser grind requirement than the double.
The single basket above (the "small" single) holds a little less coffee than the "large" single, although the lower shoulders are not "waisted" as much, so it's deceptive, and although the bottom looks slightly wider, the holes in the bottom are exactly the same size, area and number. This basket by virtue of it's reduced volume and "flatter" interior profile, does need a finer grind to avoid problems, does not give as good an extraction and is definitely more problematic to use. also the raised rim on the perimeter can make it difficult to insert into some portafilters, especially those with slightly thicker wire retainers (but push hard and they will eventually pop in, but will be difficult to get out)
Conclusion
For successful singles, try and obtain the "large" variant of the single filter basket. The easiest way to tell what you have, seems to be to compare them directly with the height of the standard double, if they are the same height, congratulations, you probably have the "large" single, it is shorter, then you will have the "small". I am sure the "large" variant can be obtained at many different places and from many different manufacturers. I can only say that, to this date, the standard Gruppo Izzo Vivi and Alex portafilter baskets are of the larger variant. It's also part of the secret why I am always so successful with the single shots, when some people struggle…I have the large single and struggle with the "small" single (90% of my shots are singles).