What is the maximum amount of RAM an app can use?

What is the maximum amount of RAM an app can use?

I am quite curious about this question so I hope for a quite detailed
answer on that topic.
What is the maximum amount of memory (in Megabytes / as percentage of the
total RAM) that an Android application (that is not a system app) can use?
Are there any differences between the Android versions?
Are there differences concerning the manufacturer of the device?
Which "side factors" are taken into consideration when it comes to
determining how much RAM an app can use?
What I have heard so far:
Early devices had a per-app cap of 16MB
Later devices increased that to 24MB
What makes me very curious:
Both of these limits are very low.
I have just recently downloaded the Android Task Manager to check my
devices RAM. What I have noticed is that there are applications using
around 30-40 megabytes of RAM, which is obvioulsy more than the mentioned
maximum RAM usage of 24 MB. So how is it calculated which RAM an app can
use? How is it possible that apps exceed that limit?
Furthermore, I noticed that some apps of mine crash with an
OutOfMemoryException when using around 40-50 Megabytes. On the other hand,
I have apps running on my phone using 100 MB and more (a friend of mine
once developed a "leaking" app :-)) that do not crash. So it obviously
also depends on the app itself when it comes to determining how much RAM
can be spared. How is this possible?
Thank you in advance for your explaination.