Android :: Png Downloaded Size Different To Server Size?

My app downloads .png files to the sd card for later use. I kept getting OutOfMemoryErrors (if anyone could explain this too, that'd be great!) and so I took a look at the sizes of the images saved to the sd card, and they seem to be roughly double what they are on the server. Why is this, and how can I make them smaller?
public void onCreate(Bundle saved)
setContentView(R.layout.namedrxnscreen);
TextView t1 = (TextView)findViewById(R.id.rxn_text1);
TextView t2 = (TextView)findViewById(R.id.rxn_text2);
TextView t3 = (TextView)findViewById(R.id.rxn_text3);
TextView t4 = (TextView)findViewById(R.id.rxn_text4);
iv = (ImageView) findViewById(R.id.rxn_image);
pb = (ProgressBar) findViewById(R.id.rxn_loading);
vs = (ViewSwitcher) findViewById(R.id.rxn_switch);