Honestly, I don't know what causes bloatware, except that the companies that make it only make money from adding new features to existing software. So rather than reconsider the original design, they just add on to what they have, without consideration for design.

I wrote custom business apps. for several years, I shot for clear, maintainable code. I also shot for elegance, meaning simplicity of design--this was all contingent on if I had time for it in the budget/schedule--and my code wasn't bloated.

Your point that commercial software now runs just as slow or slower than software from 20-30 years ago is right on, but clear code does not correlate to bloatware. You're barking up the wrong tree.