Software doublnig in size is nothing. I assume you're talking about the actual installation since the installer itself is only a little over 1mb. Going on the assumption that the installer is always a little more than half the size of the installation the last version to have a 1mb installation was mIRC 4.6, 6½ years ago. Let's see how things have changed since then:
- mIRC: 1MB -> 2MB (2x)
- Writeable media: 1.44MB -> 650MB (451x)
- Net connections: 28Kb -> 512Kb (18x)
- Hard drives: 2GB -> 80GB (40x)
- RAM: 16MB -> 256MB (16x)
- Processor: 133MHz -> 2.4GHz (18x)
I couldn't find any proper figures on software size in late 96 and now. But just think that software then used to be spanned across floppy discs or take up a fraction of a CD, and now software fills a CD or is spanned across several and it's clear to see that pretty much all computer programs have grown far more than mIRC has size-wise.
So yes, mIRC has doubled in size, but everything around it has grown a hell of a lot faster - relatively speaking you could say that mIRC has actually shrunk over the years.