Size of png file is reduced - vb.net

Question

I changed the last 4 bits of all pixels in a png image to zero. Why was the size of image reduced to less than half?. I believe png is lossless compression. Will I be able to retrieve the embedded data.(here I added 0000 to all pixels, but in actual case i need to add different data) I'm using vb 2015. The code I used is attached below.
Dim image1 As Bitmap = New Bitmap("C:\STEG\originalimage.png")
For i As Integer = 0 To image1.Height - 1
For j As Integer = 0 To image1.Width - 1
Dim value As Color = image1.GetPixel(j, i)
Dim t1 As Byte = value.R
Dim t2 As Byte = value.G
Dim t3 As Byte = value.B
t1 = t1 And 240
t2 = t2 And 240
t3 = t3 And 240
image1.SetPixel(j, i, Color.FromArgb(255, t1, t2, t3))
Next
Next
image1.Save("C:\STEG\originalimageedited.png")

Answer 1

PNG is a lossless image compression format, yes. However if it's lossless or not doesn't anything have to do with the great reduction in size. What affects it is how the compression algorithm works.
I don't know how PNG's algorithm work, but basically compression is about repeating data. For instance if the last 4 bits of every byte is 0000, like so:
1000 0000
0000 0000
1101 0000
0110 0000
...then a compression algorithm could for example express it as:
repeat 0000 every 4 bits
or:
1000 4-0
4-0 4-0
1101 4-0
0110 4-0
(4-0 indicates that it should write four '0' bits)
which might only take up a few bytes in the compressed file, while in the uncompressed file these four bits make up half of every byte.
Thus if you have 1 KB = 1024 bytes of color data and compress it like the first example above, the file size will (roughly speaking) shrink to 512 bytes + the few bytes that indicate the repeat.