As a caveat, the answer to your question is likely to vary from employer to employer. Still, we can answer in generalities. You asked a few questions,
First of all I would like to ask: is my assumption (that juniors generally get bigger raises) correct?
If we're speaking of annual performance-based increases, I don't think your assumption is correct at all. Generally, an annual increase is tied to a budget with some sort of normalizing factor. That is, a department may be budgeted 5% of their payroll for increases, and then it's up to them to distribute based on some formula or factor - for instance, people with good reviews get 6% while those with poor reviews get 4%, and the 5% default goes to "average" employees. I have never seen a case where new, or junior, employees are given larger increases by default just because they're newer.
Your premise that newer employees are learning/growing faster may be somewhat true, but it's also subjective. And it's easily counter-argued by pointing out that your employer was essentially over-paying you during your first few weeks/months as you were learning, essentially with the result that everything cancels out.
Second: can I have an idea of the percentage of raise that a junior can get after 9 months?
That's really hard to answer and will be very dependent on the employer. In many cases, there is a basic fixed rate that is meant to be slightly better than inflation (or the cost of living). But there are many, many other factors that may go into the equation.
All of the above aside, it's important to note that most employers differentiate between annual performance increases versus promotion increases. In other words, if your title is Junior Programmer and you stay in that role for 3 years, you can probably expect a small increase each year (say, 5%). But then, in your 3rd year, if you have shown significant growth, you may be promoted to Senior Programmer at which point you get a 30% increase in compensation. Generally, the process of handing out regularly-scheduled annual increases based on performance is separate from, and smaller than, a large increase given as part of a performance. And since you expressed that you're expecting to grow and learn rapidly, you may find that the increase you're expecting as a result of that growth comes in the form of a promotion rather than as part of the annual cycle.
Finally, you commented,
If my assumption is correct, I should contest this decision (individually, for my own salary of course) and ask them an ad-hoc raise.
While it's noble to contest the decision, keep in mind that for many employers, by the time the discussion about annual increases is being had with employees, the numbers are locked solid and not up for debate.