I’m a candle fan but my efforts to make my own using recycled wax have for years had a problem: they work fine when I first start burning them but as time goes one the flame gets smaller and smaller, until I’m left with a nearly useless pea-sized glow. Brand new pillar candles often do this too, although usually not as severely. This btw is distinct from the problem of candles burning a pit down into the center and then drowning, which is usually due to people not letting them stay lit long enough.
Well I finally discovered the cause: Pillar candles are often sold with the outside coated with a thin layer of clear plastic. This helps protect the candle and keep it looking good. Unfortunately they don’t always use a coating that is compatible with burning; the melted plastic gets soaked up into the wick where instead of burning cleanly it forms charred gunk like most plastics do, clogging up the wick. This is bad enough in a new candle; if you melt down scraps for reuse you have now spread the plastic throughout the body of the wax making it worthless except for looking pretty.
The moral of the story being the larger the candle, the more likely it was formulated to look decorative rather than actually burning well. Doubtless they sell more candles this way.