。
She's changed a lot since the last time I saw her.
Literal
She [topic-は] this-time-before met time [from-から] considerably has-changed.
この前 ('the other day, recently, last time') is a common phrase setting up a recent past reference point. 会った時から ('since the time we met') uses から (here meaning 'since' or 'from') to mark the reference moment from which the change is measured. 随分 (zuibun, 'considerably, quite a lot') is a versatile intensifier — slightly more written or thoughtful than とても. The past tense 変わった ('has changed') describes a state change rather than an ongoing action. A reflective sentence — the kind of remark you make after running into someone you haven't seen in a while.