This belongs to medical Astrology and can be used to understand weight problems or as a method to take ‘advantage’ of planetary influences to shape your physique.
The main indications for weight loss & weight gain :

Jupiter = Gain
Saturn = Loss

Responsible Planets :

Sun = Digestive fire
Moon = Nurturing, water, body fluids
Venus = Food, security & sweets, linked to obesity
Mars = Attitude towards exercise
Jupiter = Fats, linked to obesity, liver
Saturn = Salt & bones
Neptune = Alcohol & addictions
Pluto = Passion & transformation, Fat metabolism

Responsible Houses :

2nd house = Self esteem & food
4th house = Digestion
6th house = Digestive system

Indications in the Birthchart 

Skinny, Lean & underweight :
• ASC Capricorn are often lean & skinny people
• Saturn in 1st house or conjunct ASC
• Saturn in aspect to ASC ruler
• Northnode in 1st house
• Northnode conjunct Saturn

Chubby, curvy & overweight :
• ASC Sagittarius are often chubby or curvy
• ASC in Cancer or Taurus are often thick or full
• Jupiter in 1st house or conjunct ASC
• Jupiter in aspect to ASC ruler
• Jupiter in Caner or Taurus
• ASC ruler in Taurus
• ASC ruler in fixed sign
• Mars in Libra or cancer

Triggers via Transits

Weight gain :
• T Jupiter – ASC
• T Jupiter – Venus
• T Jupiter – Moon

Weight loss :
• T Saturn – ASC
• T Saturn – Moon
• T Saturn in 1st house = Trimming of the excess
• T Saturn in 6th = Decipline & seriousness towards diet & health
• T Pluto in 1st = Transformation
• T  Pluto – ASC ruler
• T Uranus in 1st = Change, excitement
• T Uranus – ASC ruler
• T Uranus in 6th = New diet/exercise ideas

Triggers via Progressions

Weight gain :
• P ASC in Taurus or Sagittarius
• P Sun & Venus in Sagittarius
• P ASC ruler in Taurus or Sagittarius
• P Jupiter in aspect to ASC or ASC ruler
• P Jupiter in aspect to Venus

Weight loss :
• P ASC in Capricorn
• P Saturn in 1st house
• P Saturn in aspect to ASC or ASC ruler
• P Sun in Capricorn

Sometimes you gain weight while you are still watching what you eat. Checking your transits and Progressions could help in understanding why this happens. Your body might be more prone to holding onto fats when Jupiter is making a strong aspect to your ascendant or its ruler, this also applies when Jupiter is making an aspect to the sun.
The moon is also responsible in times of gaining weight more easily, it rules the fluids in your body ; when jupiter is trine or conjunct to the moon in both transit or progression it not only brings joy and a more optimistic feelings, but it also adds to your body holding onto fluids a.k.a water retention.

However, some people are ‘blessed’ with having a fast fat metabolism, which can be seen in the birthchart, such as having mars in it’s own sign – Aries or Jupiter in a fire sign.
For those people the transits and progression are of minor influence since there genetic and astrological build is different and able to process fats more easily.