That would be because you didn't look at the details. "600W" means that it is capable of pulling 600W when driven at it's maximum rated value. But since the cooling isn't there for that means it would go "pffzzzt" in a matter of minutes so is driven at a much lower level.
That is why I mentioned the "minefield" of pre-built lights earlier, for I've seen lights on AliE saying "600w" but who's actual power draw was under 100w, "300W" lights saying they are so efficient they use 50w. The actual power used is critical, not what they claim it is (our 300w light is as good as a 450w HID but only pulls 50w from the wall" doesn't really add up, does it), for that tells you how much they are having to under-drive their LEDs to allow it to work.
If they don't provide details of actual current draw, do not think of buying them. If they recommend you turn it off every 5 hours to let it cool down, ignore it. Look at NUMBER of ratings as well as actual comments to see "flaws", look at sales numbers as the higher they are the more likelihood of a more quality product and good service. And you can always ask questions, as others will have, where buyers will give their opinions. That's the basics for AliE shopping, if you're on Wish or Amazon you're on your own because I don't trust one word on either site as there's a hell of a lot of misleading there, and you will find "branded" lights there as well as knock offs which, on paper, look to be the same as the "brand" (Not mentioning names, as the same applies across the board) insofar as power, etc, go, but what you've done is not done some basic checks and bought something bigger that is driven at a lower level than your 300w lights. It's quite possible they even pointed that out in their "ad", but you didn't look.
So contact the retailer and ask why this is, demanding all the info such as power usage and see if it matches the ad on <insert reseller site here>. If you can't yourself, and it's easy to buy a watt meter that goes into your wall socket if you don't have the skill or courage to use a multimeter with mains power going through it, then ask someone to see what the power draw is. If there's no match to ADVERTISED actual current/power draw, NOT the "600w" BS, then you have grounds for refund. Otherwise they'll just say you didn't see or didn't ask so you bought it at your own risk so, well, suck it up because you bought a pig in a poke.
One last thing. How hot does the power cable get? Is it nice and cosy? If so, replace it with a proper one capable of 3kW as it should be a standard "kettle lead" (for those of you old enough to remember the days before it became a "PC power lead") connector and that side of things is notorious for having thick insulation around a thinner cable than a "genuine" one has, leading to higher impedance, an increase in temperature, a drop in the available voltage and/or current flow, meaning your light is dimmer because the power cable isn't capable of handling the necessary so less than necessary reaches the light. You've probably got a spare somewhere, try replacing the power cable and see if there's a difference.