I believe that solely comparing MPG #s alone can be misleading, almost a false equivalency.
Todays cars and yester year's cars are not even apples to apples. Today's cars are overall much more efficient than older variants. Not only are they more efficient, they are also way safer, quieter, way more reliable... Handling, ergonomics, overall comfort, accessories & electronics... Today's cars are super luxury cars compared to any 1980s-90s era cars.
That being said... Here are some practical reasons that I can think of of why you can squeeze more out of an older Metro.
#1 - They are all basically starting around a very similar baseline in terms of overall fuel efficiency ratings. Check out the attachment to see a Side-by-Side comparison I made between the four cars and you'll realize they're all very close to each other in fuel efficiency.
#2 - A quick look the stock curb weights shows the Geos are 1,200-1500 lbs. lighter. So the effects of any modification will tend to be "amplified" in comparison to an 'almost-twice-as-heavy' car.
#3 - They are cheaper!
This may sound funny, but I bet you that almost no one who just paid $20-30K + on a modern Ioniq/Prius/Insight is willing to do 1/4 of the things required to get that great mileage out of an older beater GEO. (myself included). With the older beaters its actually fun to tear stuff apart and drill some sheetmetal screws right into your body panels.
Add all these together and you have a good
starting point for how you can make these Geos more efficient than todays modern cars MPG wise... IMHO