While it differs for different cultivars, I don't think "a lot more" is true in general. For the best tasting commercial varieties, I think they are roughly comparable.

Hayward Kiwi (the most common commercial variety in the US) are harvested fairly hard at about 6 Brix (approximately percent dissolved solids assuming those solids are sucrose) and ripen up to about 12-14 Brix.[1]

Strawberries are usually 8-10 Brix, but a peak of the season fully ripe (and delicious) modern variety like Seascape or Tristar might be 12-14 Brix[2], the same as a fully ripe kiwifruit.

[1] https://thebenjamin.wordpress.com/2013/11/15/harvesting-kiwi...

[2] Sorry, I'm not immediately finding a good cite for this one. I've personally measured a bunch, though.

I suppose it depends strongly on the particular cultivar. I know I was surprised by the difference when I looked it up a couple of years ago. I was doing a keto diet at the time, so you read a lot of nutrient tables.