The 58kg category is made up of 19 lifters, with 11 in the A group and eight in the B group – there are only 10 lifters in the 63kg category.
In a statement British Weightlifting said: “Zoe is a young athlete in her first Olympic Games, and the most important thing for her long-term development is that she has a positive experience and lifts to her potential.
"In the 58kg group she is in a very competitive B group including (Hidilyn) Diaz, ranked sixth in world in 2012, and (Maria Alexandra) Escobar Guerrero, ranked four in the world in 2012.”
Smith broke six British records at the final Olympic trials in May and will be looking to repeat that at the lower weight category.
They added: "This gives Zoe the opportunity to challenge those athletes that are around her in the world rankings and go for PBs (personal bests) in the weight category she will be most competitive in in the future.
"Zoe will be looking to set British records at 58kg.
"You will see from the 63kg start list that although Zoe would have made the A group, she would have ended up following herself (her three snatch lifts would have been consecutive).
"Zoe sees her long-term future in the 58kg category and has worked hard with the BWL (British Weightlifting) nutritionist to ensure that she performs well at that weight. Her focus when the Games were announced was always to lift at 58kg, and we have stuck with that plan."
British Weightlifting performance director Fiona Lothian, said: "The plan was to lift at 58kg, and we’ve stuck with that looking forward to Rio 2016 Olympic Games.