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En el wiki de lazarus aclaran lo del Smart link

Smartlinking

The base principle of smartlinking is simple and commonly known: don't link in what is not used. This of course has a good effect on binary size.

However the compiler is merely a program, and doesn't have a magic crystal ball to see what is used, so the base implementation is more like this
The compiler finely divides the code up in so called "sections".
Then basically the linker determines what sections are used using the rule "if no label in the section is referenced, it can be removed.

There are some problems with this simplistic view:
virtual methods may be implicitely called via their VMTs. The GNU linker can't trace call sequences through these VMTs, so they must all be linked in;
tables for resource strings reference every string constant, and thus all string constants are linked in (one reason for sysutils being big).
symbols that approachable from the outside of the binary (this is possible for non library ELF binaries too) must be kept. This last limitation is necessary to e.g. avoid stripping exported functions from shared libraries..
Another such pain point are published functions and properties. References to published functions/properties can be constructed on the fly using string operations, and the compiler can't trace them. This is one of the downsides of reflection.
Published properties and methods can be resolved by creating the symbolnames using stringmanipulation, and must therefore be linked in if the class is referenced anywhere. Published code might in turn call private/protected/public code and thus a fairly large inclusion.

Another important sideeffect that is logical, but often forgotten is that this algoritm will link in everything referenced in the initialization and finalization parts of units, even if no functionality from those units are used. So be careful what you USE.

Anyway, most problems using smartlinking stem from the fact that for the smallest result FPC generally requires "compile with smartlinking" to be on WHEN COMPILING EACH AND EVERY UNIT, EVEN THE RTL

The reason for this is simple. LD only could "smart" link units that were the size of an entire .o file until fairly recently. This means that for each symbol a separate .o file must be crafted. (and then these tens of thousands of .o files are archived in .a files). This is a time (and linker memory) consuming task, thus it is optional, and is only turned on for release versions, not for snapshots. Often people having problems with smartlinking use a snapshot that contains RTL/FCL etc that aren't compiled with smartlinking on. Only solution is to recompile the source with smartlinking (-CX) on. See buildfaq for more info.

In the future this will be improved when the compiler will emit smartlinking code by default, at least for the main targets. This is made possible by two distinct developments. First, the GNU linker LD now can smartlink more finely grained (at least on Unix) using --gc-sections, second the arrival of the FPC internal linker (in the 2.1.1 branch) for all working Windows platforms (wince/win32/win64). The smartlinking using LD --gc-sections still has a lot of problems because the exact assembler layout and numerous details with respect to tables must be researched, we often run into the typical problem with GNU development software here, the tools are barely tested (or sometimes not even implemented, see DWARF standard) outside what GCC uses/stresses.

The internal linker can now smartlink Lazarus (17 seconds for a full smartlink on my Athlon64 3700+ using about 250MB memory) which is quite well, but is windows only and 2.1.1 for now. The internal linker also opens the door to more advanced smartlinking that requires Pascal specific knowledge, like leaving out unused virtual methods (20% code size on Lazarus examples, 5% on the Lazarus IDE as a rough first estimate), and being smarter about unused resourcestrings. This is all still in alpha, and above numbers are probably too optimistic, since Lazarus is not working with these optimizations yet.

Enlace al wiki: http://wiki.lazarus.freepascal.org/Size_Matters