read this book - http://letrongngoc.tech.officelive.com/Documents/TheCProgrammingLanguageSecondEdition.pdf - especially the part about sizeof, better read the whole book.
It are the basics of programming explained by the creators of C and UNIX. It will take a while to go through the book but you will learn so much from it ...
If you really want to learn why the executable got smaller you need to learn about compiler building, assembly and that kind of stuff. Not for the faint of heart 
With the tool objdump.exe you can see the assembly language generated by the arduino compiler and see what has happened.
[in short, changing datatypes from int16 to int8 size gave the compiler the chance to use other assembly instructons to do the code]
If questions remain, please feel free to ask them.