A lot of the work with most patches is the coding, finding a bug, or implementing a major change might only be a few hundred K or even less, time has to be spent to QA any change, also Squad uses a version control system that allows them to be working on any and all versions, so work done during the 0.22 working phase might be on features for the 0.23 or even 0.60 Not a lot of the KSP is massive graphics. For a good example, sit down with a text file editor and try and write an essay that comes to 10Megs. Let me know when you are done. To give a good hint, the Bible is about 1.2megs in text. The software/games industry is well known for not paying staff for the 60-80 hour weeks they can put in, and the 80-100 hour weeks they can work to reach a release, you may have bought the game, but unfortunately you are not the boss, and worse off you have no knowledge of the pay conditions or hours. Unfortunately everyone has different expectations of what should and shouldn't be worked, and also resources will be used by Squad as they become available. Also note that even small changes you can't see can require a lot of code. See this post about how changing the end flight button effects more then a simple 2 second fix. As each part is added the physics become s much more completed, think the Fibonacci sequence , First part is easy 1 caculation, Second part has 3 calculation, third part has 5, forth has 10 etc.. On the Fibonacci sequence the 500th number is 139423224561697880139724382870407283950070256587697307264108962948325571622863290691557658876222521294125, Thats a lot of caculations. I don't see why 2 years would be wrong, most good games take 3-5 years to be developed, as you add each little bit everything gets more complex, and counter intuitively as you ad more ppl the work required to move forwards increases as well at a relatively low number.
