The reason is that, when you revise those notes, it will take a lot of time to revise and those will be like theory only so you will not enjoy them much. Also, if you need to find some specific point or want to have a look at it you will keep on struggling through the pile. If your notes are short and crisp you will find that point easily.

Now what should one write on that blank paper? For physics I recommend writing important formulas first then some concepts and method for solving a specific type of question. For example, in electrostatics you can write all formulas for electric field and potential. Also, you can write about charge distributions and capacitance stuff. In mechanics you can write some method of solving a particular set of questions.

For physical chemistry, write formulas. For inorganic chemistry? I do not recommend making notes for inorganic chemistry. Make your NCERT your short notes page. You will see a lot of empty space in NCERT pages. If you find something which is important or there is some new compound name or some reaction or even some priority order of elements such as density and all and it is not in NCERT then write it in the blank spaces in header and footer on the concerned page only.

For organic chemistry one should write named reactions only or some important mechanism or some concepts or points that have to be remembered. For example- Alkenes is more reactive for hydrogenation than alkyne. This is a fact, and you have to remember it. So, write it in the notes of the chapter including alkenes. One should also write reagents and their one-liner reaction in short. These are very helpful. DO NOT FORGET TO WRITE EXCEPTIONAL CASES IN ORGANIC AND INORGANIC CHEMISTRY.

For mathematics, you can write some points to remember type thing. I strongly recommend making good notes for conic sections. I only made notes for conic sections and one page full of trigonometry formulas. That’s all! Use good diagrams and write properly all formulas and methods of solving the problem in conic section notes. This contradict with the things I told above but yeah; I suggest you make good notes for conic section. 

Does one have to make notes for every chapter? No. You do not need to make notes for every chapter. You can club some topics or skip if you think it does not need revision again and again. Also, you need not make notes as soon as a chapter gets over. Make notes on month end. In this way you will revise whatever has been covered in the entire month.

If I talk about revising the notes, you should do them at your ease. The main aim for short notes is to revise the topics which you have forgotten and have a compiled info for important stuff at one place.

You should also write important questions numbers in your short notes. I used to do that. Be very specific. Example: Topic- Name of the book/module/dpp/sheet e.t.c – Q 1,2,45,69…This helps to set up a compiled question set which you can use to revise in your revision time.

This is how you make short notes which are effective. Make sure you revise them time to time and keep updating them whenever you find something worth to add on. This method will surely help in revision and boost your rank.