In my experience, exploding blocks will generally make the files larger, not smaller.
I think i explained a little bit wrong. Goal is to clean the drawing.
When i'm done i put the result together in one block.
So actually your intent is NOT to ensure the file is as small as it possibly can be. It's rather to make it a flat file without any reused blocks.
Just so you know, that's not going to make the drawing smaller than it "could" have been. The last step (place everything into one block) will definitely increase the size by a small amount, while the first step (explode all blocks) would "most probably" increase the size by a huge amount (depending on how blocks were used to begin with).
Using a block can make the drawing multiple times smaller "if used correctly", but placing the entire drawing inside one single block would actually make it slightly LARGER. The only time a block makes a drawing smaller is when that same block is placed 2 or more times in the drawing.
Think about it ... the block defines the linework once. The inserts reference the linework into different places without actually making new copies of each line. Thus before exploding your drawing might look like this:
Block Definitions
Block1
Line1 X,Y,Z to X,Y,Z Layer Color etc.
Line2 X,Y,Z to X,Y,Z Layer Color etc.
Line3 X,Y,Z to X,Y,Z Layer Color etc.
Model Elements
Block1-Insert1 @ X,Y,Z Rotate Scale Layer Color etc.
Block1-Insert2 @ X,Y,Z Rotate Scale Layer Color etc.
Block1-Insert3 @ X,Y,Z Rotate Scale Layer Color etc.
Block1-Insert4 @ X,Y,Z Rotate Scale Layer Color etc.
After exploding that same file becomes something like this instead:
Model Elements
Line1.1 X,Y,Z to X,Y,Z Layer Color etc.
Line1.2 X,Y,Z to X,Y,Z Layer Color etc.
Line1.3 X,Y,Z to X,Y,Z Layer Color etc.
Line2.1 X,Y,Z to X,Y,Z Layer Color etc.
Line2.2 X,Y,Z to X,Y,Z Layer Color etc.
Line2.3 X,Y,Z to X,Y,Z Layer Color etc.
Line3.1 X,Y,Z to X,Y,Z Layer Color etc.
Line3.2 X,Y,Z to X,Y,Z Layer Color etc.
Line3.3 X,Y,Z to X,Y,Z Layer Color etc.
Line4.1 X,Y,Z to X,Y,Z Layer Color etc.
Line4.2 X,Y,Z to X,Y,Z Layer Color etc.
Line4.3 X,Y,Z to X,Y,Z Layer Color etc.
So the more copies of the same block you explode the more new lines you're actually placing in the drawing's model.
If you then go and make a new block of all of them you're simply going to end up with something like this
Block Definitions
Block1
Line1.1 X,Y,Z to X,Y,Z Layer Color etc.
Line1.2 X,Y,Z to X,Y,Z Layer Color etc.
Line1.3 X,Y,Z to X,Y,Z Layer Color etc.
Line2.1 X,Y,Z to X,Y,Z Layer Color etc.
Line2.2 X,Y,Z to X,Y,Z Layer Color etc.
Line2.3 X,Y,Z to X,Y,Z Layer Color etc.
Line3.1 X,Y,Z to X,Y,Z Layer Color etc.
Line3.2 X,Y,Z to X,Y,Z Layer Color etc.
Line3.3 X,Y,Z to X,Y,Z Layer Color etc.
Line4.1 X,Y,Z to X,Y,Z Layer Color etc.
Line4.2 X,Y,Z to X,Y,Z Layer Color etc.
Line4.3 X,Y,Z to X,Y,Z Layer Color etc.
Model Elements
Block1-Insert1 @ X,Y,Z Rotate Scale Layer Color etc.
Of course if no blocks in the drawing is inserted more than once, then exploding should reduce size by a slight bit as the definitions and the references would get removed. But each block which is inserted more than once would most probably cause an explode of it to increase the file's size.
The reason you see large file size in this case is due to the added stuff on top of those blocks, not because they're blocks - but because some extras are attached to them. These attachments usually gets attached to those Block-Inserts instead of the Block Definition - that's why they grow the file near exponentially (they're copied multiple times over). My guess would be if you can get rid of these attachments you'd be able to reduce the file's size tremendously, even much more so than simply exploding everything.
However, I'm talking "theoretically" - I'd need to see the original file to be sure.