Getting more or less: adjective + ra / lên / đi / lại

To say something is getting bigger, hotter or thinner, put a direction word after the adjective. Lên (up) and ra (out) say it's growing: Trời nóng lên rồi. It's getting hotter. Dạo này chị khỏe ra nhiều. You've got much healthier lately. Đi (away) and lại (back) say it's shrinking or slowing: Ông ngoại yếu đi nhiều. Grandpa has got a lot weaker.

Roughly, lên goes with rising and growing (lớn lên, grow up; cao lên, get taller), ra with filling out or looking better (mập ra, put on weight; trẻ ra, look younger), đi with losing something (ốm đi, get thinner; già đi, get older; xấu đi, get worse) and lại with drawing in (chậm lại, slow down; ngắn lại, get shorter).

Many pairs are fixed by habit, so learn them as you meet them. A few take either word: mập ra and mập lên both mean putting on weight.

In the South ốm means thin, so ốm đi is "get thinner". In Hà Nội that's gầy đi, and putting on weight is béo ra.

English "get" or "become" needs no word of its own here: the direction word already says it's changing.