Basic question about size_t and ulong

[WhatMeWorry]

Quoting the D documentation:

size_t is an alias to one of the unsigned integral basic types, 
and represents a type that is large enough to represent an offset 
into all addressable memory.  And I have a line of code:

size_t huge = ulong.max;

dmd GC.d
GC.d(29): Error: cannot implicitly convert expression 
`18446744073709551615LU` of type 'ulong` to `uint


Isn't ulong an integer? And isn't memory addresses 64 bits long?

size_t huge = uint.max;  // compiles

works but now I'm just curious.  I was just seeing what is the 
largest dynamic array I could create.