No- Chill is a success when the sealed container has been effectively sterilized* by near- boiling wort. If you don't completely fill it or ensure that the entire interior surface has been subjected to sufficient heat for long enough to render all microorganisms dead then the process fails and growth becomes evident later with krausen and a bulging cube, as was the case for the OP. That does not necessarily mean the cube must be completely filled for the technique to succeed- air/ head space in the cube is no problem at all provided that every surface has been effectively heat- treated.
To do this, of course the cube should be cleaned and sanitised, add the wort then seal it, turn the cube on its side and rotate a few times as it cools to ensure complete heat treatment over all surfaces. When treated this way the cube will develop some pressure, with the air that does remain sometimes the contraction on cooling is excessive but after a few goes you soon a feel for closing it with some air present and not ending up with a collapsed cube which may rupture at a fold. A tap can be fitted though, it is jolly handy when emptying or even fermenting in the cube but it is also quite helpful for relieving the pressure that builds up in the initial cooling phase while it is rotated and moved around. You don't need to evacuate all that air, particularly if the cube is far from full, however partly- filling a cube and not treating every surface effectively is unfortunately likely to fail, so I recommend using a tap.
However, the lower the volume of wort in the cube then the lower the area that can be treated before the wort temperature becomes ineffective. The interior of the cube (plus the lid, seal and tap if fitted) has to be heated with near- boiling wort for some time to render any bugs present as good as dead.
* As above, I use the term sterilized with some caution- the overall effect is far better than sanitized however it probably won't meet the sterile specification if analyzed in a lab; for our purposes though the wort is as good as sterile. A helpful analog is UHT and Pasteurized milk, No- Chill is equivalent to UHT Milk- provided it remains sealed it stores indefinitely and without refrigeration, while pasteurized milk certainly won't keep that way for very long at all. Open up UHT though and all bets are off.
Hope that helps! :icon_cheers: