I get they are largely Agosta class boats with a reactor section inserted to replace the machinery section, but I genuinely do not understand how they managed to make such small nuclear submarines.
At that point its got to be almost more expensive to miniaturise everything compared to making a larger, more capable SSN. Still interesting boats though.
Less power mainly, I would imagine (limited to 20-25 Knots max, depending on sources). I also think miniaturization was not that much of a problem, since the US managed to fit a nuke reactor in a 400T sub more or less a decade earlier in the form of NR-1 (granted it wasn’t a military submarine)
Rubis are budget SSNs with questionnable crew comfort, and they also pale in comparison of others when it comes to ammo capacity (mix of 14 torps and AShMs in total).
Thankfully, Suffren exists X)
Still, having a SSN almost as short as a type VII seems fun.