On some 32bit systems PHP will take the memory settings like 2000M or 2G and convert it to the integer number of bytes by not performing a boundary check. A number starting at 2G or 2048M will be -2147483648 bytes then.
Some PHP versions cap this at the top, so it won’t go into negative numbers (that is the 32 bit signed integer limit).
If you want to achieve the maximum possible number of bytes on such a system then, use 2147483647. This is equal to two gigabytes minus one byte.
Alternatively if you need to deal with large data, consider a 64bit system.
Additionally you should consider the following:
According to the PHP manual, the memory_limit setting is the more important one. If it does not offer enough memory, the post-data size-check then will pass, but PHP would not have enough memory to actually handle the post-data. You will get another error than, that the memory exceeded. So when you configure your PHP, take care that post_max_size is smaller than memory_limit.
In your example the memory_limit is 128M, so it can not process post-data of a size larger than ~128 Megabyte.
(This blog post shows what can happen and how large memory settings on 32bit and 64bit systems behave)