Slashing swords as you all know required a lot of strength to deliver lethal damage, typical sword slash produced a lot of kinetic energy, which means that even if armor managed to deflect/stop such hit, there will be still noticeable blunt force trauma dealt with every hit. As noticed thorough our history by many, slashing and cutting was not considered as the most lethal type of attack, and it was definitely not the most effective one considering amount of energy soldier hadto spend to kill his enemy. This reflects into only moderate normal damage dealt by all slashing weapons. From tactical perspective, longswords due to its length would provide soldiers with a bit more standoff against their opponents using the shorter swords, therefore these swords would also guarantee small defense bonus.
Swords like Kopis/Falcata are also treated as slashing, yet they deal a bit more normal damage due to their shape, they also deal less trauma damage due to less weight. Also, their length (65cm) means they get minimal defense bonus.
Thrusting swords are completely different. Thrusting attack is the most lethal type of attack you can do to human body, and you don't even need too much strength to stab deep enough to be very lethal. Thrusting attacks are very efficient, no surprise Romans preferred them over slashing attacks. Yet, because these swords were usually thrusted underarm, their kinetic energy was relatively low (approximately half what is seen with slashing swords), therefore they don't produce significant blunt force trauma. (you need about 100 joules to fracture bones). Anyway because of their speed and energy effectivity, they are able to land several attacks in time soldier with longsword would score one hit. yet, they have to rely on their shields for defense, as shortswords don't provide that much of a defense bonus (even though difference is not big, as Gladius used by Republican Legions was not that much shorter from Celtic Longsword (75cm vs 85cm). Yet, there is another advantage shorter thrusting gladius have. It was much easier to manipulate with it even in confined space, where legionaries could use it to thrust into unprotected areas like armpit, femoral artery, groins etc. because of this, all thrusting shortswords get special melee defense bypass (enemy melee defense is reduced against them) which gives Romans advantage when fighting unarmored enemies.