Very simple. You can't tell bud weight by simply looking at it. This gets brought up ALL the time and people continue to believe you can. You can't...period.
Different strains have different densities.
So how do I tell?
A) Buy a cheap $15 electronic scale on ebay or some other online store.
B) Buy a letter scale. They cost like $1-3
C) Use american nickles (they weigh 5 grams) and compare them in a bag with the bag of bud.
Example: You buy a 1/4 and want to know if you got majorly ripped. Take the bud and move it to one corner of the baggy. Place 3 nickles (15 grams) in the corner of another baggy. Compare weight of the two baggies.
Obviously B and C are not as accurate (specially C) but bottom line....
You can't tell the weight of bud with your eyes.