or is it only the size and how you diffuse it?
If you're using a panel with a large quantity of LED lights.. like 600 tiny individual leds on the panel, it will be more diffuse and soft than if you were using a single large bulb with a comparable brightness value of the panel.
A bare LED (I.e. non-diffused) will be harder than a single large source of the same size due to the fact that it is an array of lots of hard lights.
If you have one bright light you'll get a very distinct shadow from the hard light.
A 3x3 panel of 600 bare led bulbs will be a harder light source than a 3x3 panel single light source, all other things being equal. Now, the difference may or may not be marginal and it's difficult to compare directly because in the real world you'll be unlikely to find a situation where you need to, nor where you will have a situation where all things are otherwise equal.If you have one bright light you'll get a very distinct shadow from the hard light.
If you have a 3x3 foot panel of 600 led bulbs you most certainly will not get the same hard shadow effect.
A 3x3 panel of 600 bare led bulbs will be a harder light source than a 3x3 panel single light source, all other things being equal.