Not only are the conductors in most extension cords thinner, the contacts inside are not robust. They aren't rated for continuous duty at their rated current in the majority of cases.... you may have seen the phrase "temporary power tap" on multiple outlet extension cords and plug strips and the intermittent duty ability of the contacts is the main reason for this.
For high drain appliances, the rule is one size thicker than the appliance cord. So, if your space heater has a 16AWG cord, the proper extension cord would be 14AWG. The bigger cords have more robust contacts and the increased wire size minimizes resistance which lessens power lost as heat along the line.
For power strips, it's different: no more than about 8A per outlet even though the total rating may be 15A. The outlets on most strips are designed with lower drain electronic equipment in mind for the most part. While fixing computers, I have seen space heaters plugged into strips and the scary thing is the plug has melted itself to the strip and is inseparable!
The heater was only drawing 12A while the strip was good for 15A total but clearly each individual plug cannot handle the full load. To my dismay, this is almost never mentioned in any handbook that comes with one of these strips.
