I'm 4 foot 11 inches and weigh 45kg. I need to pass medicals for immigration purposes, will I be heavy enough?

No weight cutoff. The US immigration medical exam does not have a weight cutoff. However, if a patient has recently lost a substantial amount of weight, for example due to active TB infection, then he or she would need to be treated for that first in order to proceed with the immigration process.
Weight? I believe the medical exam is to see if you're healthy or not. Weight shouldn't be a factor unless you are outside the normal range for someone your size. Based on your height it appears to be a normal weight.