this is a exploit of the compression algorithms to make a small zip that will extract into extream amounts
their are more ways and better ones than this one but i will only show how to make a simple 1k = 1m ratio

Code:
open and type the null character (alt + 255)
press ctrl + a then ctrl + v a couple times to make some null bytes
if u have a hexeditor make the hex 00 for about 50 kilobytes
now make several copies of a.txt and name accordinly 
open cmd.exe
type copy /b *.txt b.txt
now every copy is made into a super copy and repeat 
once u have a nice empty big text file like 1gb
put it in a zip archive
because of the simple construction of the file
1gb of null bytes
the zip is only 1 mb in size and can really annoy freinds
for added fun hex edit the zip and u will see a bunch of hex 5555 
just add some more and the file will expand amazingly 
make sure to not open this after

BoBeR_182

Basically:

%00%00%00%00%00%00%00%00%00%00

becomes

10x%00

Except on a larger scale

credit goes to Bober from EH