This is because some string libraries like SSS don't have legato in the main patch. CSS has all articulations in 1 patch, so it will take more ram. You can disable the articulations thoughI disagree with CSS being ok for a lower ram computer in pro use.
Easy to notice on youtube vid.s, while most lib.s start of around 400-500mb ram load CSS is near 800mb. This also goes for Cinestrings if I remember correct.