Use linker "--relax" option.

The "relax" option enables the linker to convert certain "call"
instructions to the smaller "rcall" instruction.  This reduces the
size of the resulting binary.

Marlin/Makefile

 CFLAGS = $(CDEBUG) $(CDEFS) $(CINCS) -O$(OPT) $(CWARN) $(CEXTRA) $(CTUNING)
 CXXFLAGS =         $(CDEFS) $(CINCS) -O$(OPT) -Wall    $(CEXTRA) $(CTUNING)
 #ASFLAGS = -Wa,-adhlns=$(<:.S=.lst),-gstabs 
-LDFLAGS = -lm
+LDFLAGS = -lm -Wl,--relax
 
 
 # Programming support using avrdude. Settings and variables.