Im kinda skinny in body size. bcos of that, my size is 32A for all my bras in wardrobe. 

When i wear bra sometimes it gaps quite a bit at the top of the cups, u can totally see it from the side if im just wearing my bra or a moderate low cut shirt that is not skintight. Im wearing the staps as tight as possible n as for the back clasp of the bra it's set to the tightest setting. It actually feels too tight and slightly uncomfortable but the top part of the cup is still gapping. It's not tight to my skin at all, I can easily slide 3 fingers inside the top part of the cup to fill up the gap... 
This big gap really annoy me at the beginning whenever i wear low cut bcos ppl that r taller than me can actually see my bra or even my nipple if they've the intention. However, i slowly found out actually there is nobody really care bout what i wear when i walking down the street so i gain the confident to wear it regularly. 

Last sunday after hangout with frens i back home alone from KLCC lrt. i was wearing a moderate low cut summer dress.There is a guy, guess is foreign worker follow me from MNG til the lrt station. As the klcc is so crowded so i didnt give a damn. After i got the ticket n the train arrives i then get into the train.The outbound train from klcc is so pack n after the train start moving i realise he is standing just beside me
we were very close,he is facing my side. i start feeling something is not right but i couldnt do anything as i cant even move my body. After a few stations i realise he is keep staring down on my chest like nobody business.   immediately i tried to lift up my front collar where it's level with my bra cup top to cover the gap, i was so helpless, both hand carrying bags n stuffs. lifted up n it drops again n again. I wanted to hold my collar but i failed. My pony tails is killing me that day, gave him clear view of my chest

I stare at this perv n he didnt feel nervous at all. he smiles n tell me he saw my nip... in malay , i saw his HAND IS INSIDE HIS POCKET SCROLLING.
I suppose to get down at kelana jaya but i end up got down at asia jaya.
Lucikly he didnt follow me.

I wish i can find a solution for this.

is there any size smaller than 32A?

yes but kinda rare... 32A slightly helps me look feminine, 32AA will be like an airport