There are three things that cost you when buying skis and boots.
1.) How BIG you are.
a.) Weight primarily
b.) Height secondarily. A taller person has a higher center of mass (CM) which may effect your application of forces into a ski depending on technique and form.
2.) How GOOD you are and
3.) How aggressive you are.
A 300 lb. beginner will need to spend more money when buying a ski so that when on the slopes there is a response of the ski to efforts to pressure it and some torsional integrity when put on edge. Likewise a very good skier knows how to put pressure into a ski, to work it as it were, so that there is a response action to the input of energy. While an aggressive skier is usually a pretty good skier, perhaps the most aggressive skier I've ever seen was a fellow who had been on skis only about 10 times and he slammed into bumps at speed on his first run of the season breaking his pole not 50 yards down the hill. No form to speak of but jumping every chance he got!
So, to get the same response action out of a ski that your 40 lb. heavier cohort expects you simply do not need to purchase as expensive a ski.
For example, just a few seasons ago Volkl had skis for sale called Supersport FIVE STAR and SIX STARs. The Six was particularly stiff and I used to ask potential customers if they were linebackers in the NFL (National Football League as I understand you are from Europe!). Otherwise I'd steer most of them to the Five Star. Much better for them and more money left over to purchase a lift ticket.
Now surface area comes into play in terms of holding you up on powder too. A bigger skier will benefit from a wider ski. Floatation will need to be wider to get one more float. What I call extra-wide midfats range in width. You might get a similar amount of float on a ski with a 74 mm. mid body as a heavier skier would with a 76 mm. ski. In many models, though not all, usually a wider ski from a manufacturer is costlier as well. This may parallel performance but also parallels energy into the ski. This is usually a good thing but sometimes a particular ski, that is wider, presumes you are a good skier and therefore more likely to go off-piste. That is not to say that an intermediate would not like to go there too but he'll have to buy a ski that is wider but hopefully not too stiff.
There are so many fine products on the market now there is no doubt the right ski for you waiting for you to discover it. Finding a salesman who is able to process the question you ask, that is the balance between weight and response characteristics of a ski, and help him to help you to buy a ski by profiling yourself you should get to the slopes with a ski ideal for most of your uses.
So, in summary, weight is a very significant aspect of how a ski will perform for you. In the end you get to spend less money than your heavier cohorts to achieve the same responsiveness and performance out of a ski.
(Not sure why the system calls me PRIVATE USER but EJL has been my username.)