That's が vs. は for the subject, not を vs. は for the object. Unlike が vs. は, を is the most common for the object, so は mostly has a nuance of "contrastive marker", as the OP wrote.
Think about a situation where you are asked "Do you watch movies or TV dramas?". The answer 映画は見ます means "I watch movies", connoting "but don't watch TV dramas". This is the meaning of "contrasting".