Is selling 700-gram containers of strained yogurt, after they were 1000 grams, and at almost the same price as the larger container, considered a reduction in weight or deception, and is it permissible or forbidden?

There is no harm in selling a commodity by its true weight as written on the packaging (700 grams) or as known to the buyer. What is forbidden is deception in weight, such as writing an unreal weight on the packaging or deluding the buyer into thinking the weight is greater than the true weight.