What do women wear under Abaya in Saudi Arabia? well, there are as many answers to this question as to the number of women wearing Abayas in Saudi Arabia.
What do they really wear depends upon several factors like weather, mood, fashion, personal preference and bla bla. You cannot generalize that women wear that particular dress under Abayas.
What women really wear under abaya also depends upon the mindset to which she belongs. A more religious girl or woman will be covering her hands with the gloves, will be wearing a veil upon her face, and wearing a full dress before wearing abayas.
If they are wearing jeans shirt, it will be a fully covered dress so that if some part of abaya gets opened due to the wind or while walking, no part of their body is exposed to outsiders. You can easily identify such women in the malls.
Women who are a bit open-minded or some women who belong to western countries wear a less covered dress.
I don’t think there is anything wrong with wearing whatever they want to wear. Let’s say if a girl is going to gym she is not required to wear a full-fledge heavy dress.
She can wear her exercise dress and go there. Wearing abaya is still a cultural obligation in Saudi Arabia but there is no rule regarding what to wear under the abaya. Some women now wear a fully covered dress in the public and avoid wearing an abaya.