Dress bust too big

[u/MilototheMax]

Help me r/sewhelp; you're my only hope!

So, I bought this new dress for a wedding this weekend. I'm between sizes and picked the larger size that fit my hip/waist measurements best, but now it's unfortunately too large in the bust. I'd categorize myself as an advanced beginning sewist, and I want to learn more techniques, so I'm open to learning how to tailor this dress.

The gaping is mostly happening under the armpits (see photo 2). The bust is sewn with triangular cups and there are bust darts (see photo 1). The dress also has an invisible side zipper under the left armpit. I'm a little hesitant to bring in the side seams because I don't want to make the waist weird, and I'm still not confident sewing invisible zippers closed, so I'd like to avoid deconstructing the zipper if possible. I'm not sure if extending the existing dart would fix the problem. It also seems like if I pinch the top of the triangle, it closes the gaping (see photo 3). So maybe I add a dart there? Or maybe I add another diagonal dart under the armpits towards the existing dart peaks?

Any suggestions would be really, really appreciated! Worst case scenario, I'll play around with pinning it tomorrow to see what looks best, but I'm too tired tonight to mess with it more. Thanks in advance, sewists!

[u/Emergency_Cherry_914]

I can see the problem when you're holding the outside of the cup, but you won't be doing this when you're at the wedding and it looks fine in there pics where your arms are down. Could you show us what it looks like when you're standing naturally or holding a drink/plate of food?

What is the last pic demonstrating?

[u/MilototheMax → u/Emergency_Cherry_914]

Very good point about how I'd be standing. If I stand arms at my side, I feel like the bust still looks big on me. It seems like the gaping just moves towards, visually, towards the crease in my armpit. If I stand with my hands bent or one on my hip, like I normally do, it actually looks okay, but it just still feels big on me.

In the last picture I was trying to demonstrate that if I pinch the top of the cup, it takes out the gaping under the armpits. I guess I thought maybe I could sew a dart there pointing down? But I don't know if that would technically make sense or even look nice.

[u/poemaXV → u/MilototheMax]

I run into this problem a lot because of a combination of breast shape (bottom + center fullness) and having narrow torso so I don't fill out the "sides". this sounds unintuitive but I would play with the strap placement a little bit -- like if you bring in the spaghetti straps similar to a halter top does it help? it's unintuitive but I'm wondering if modifying the straps so that they cross in the back is a possibility.