Beacuse they were very small people with small brains that could only fit small code inside their head. Seriously tho, your program can be as bloated as you want to make them. Reasons why modern sofware often takes up more space is down to many reasons. Ultimately storage space is not really an issue on computers nowadays so less time is spent worrying about it. Computers and programs are also way more complex now. Theres nothing stopping you from producing tiny programs in C tho.
