You have 4 defsize commands but only the first one (or maybe the last one - I forget which) counts. I'm guessing you want the total size of the shape to be 80x130, in which case you need something like this:
shape main{
noshadow = true;
fixedAspectRatio = true;
defsize(80,130);
startpath();
moveto(0, 0);
lineto(50, 0);
lineto(50, 35);
lineto(0, 35);
lineto(0, 0);
endpath();
setfillcolor(100, 100, 50);
fillandstrokepath();
startpath();
moveto(0, 35);
lineto(0, 75);
lineto(50, 100);
lineto(50, 35);
lineto(0, 35);
endpath();
setfillcolor(255, 255, 255);
fillandstrokepath();
startpath();
moveto(50, 35);
lineto(50, 100);
lineto(100, 75);
lineto(100, 35);
lineto(50, 35);
endpath();
setfillcolor(100, 100, 50);
fillandstrokepath();
startpath();
moveto(50, 0);
lineto(100, 0);
lineto(100, 35);
lineto(50, 35);
lineto(50, 0);
endpath();
setfillcolor(255, 255, 255);
fillandstrokepath();
}