Thanks for the thread OP, but this is not true, and it is very misleading for people who are not familiar with technical outerwear:

The thickness of a jacket is really the biggest factor in warmth.

The insulating properties of the material itself is the biggest factor in warmth; not the thickness of the jacket.

Up until recently, down was the warmest insulation in existence. But fill power is important to note when comparing down jackets. A thick, bulky 600 fill power parka is not necessarily warmer than a thin, lightweight 1000 fill power jacket; in fact the opposite is likely true. If you are comparing two jackets with the same fill power and shell material, then you would be correct that a thicker jacket would be warmer.

If you have a down jacket compared to a synthetic jacket of similar thickness/weight, then 99% of the time, the down jacket will be warmer. (AFAIK Patagonia’s PlumaFill used in their new Micro Puffs has the best warmth to weight ratio, surpassing that of down.)

Another good thing to mention is the performance of down vs synthetic insulation. Put simply, down does not work when it gets wet, while synthetic still insulates. They do make waterproof breathable down jackets, but they are expensive and pretty rare.

Down is great in cold, dry conditions, especially for situations where you aren’t too active, like watching a parade or sporting event. Synthetic is great if there is a chance of light rain, or in more active situations.

Finally, if you’re wearing your puffer to layer under a waterproof shell, it may be better to skip the hood. But if you’re wearing it as an outer layer, I definitely recommend spending the extra money for a hood!