Need help determining proper size/style implant for my desired result. (photo)

Not HUGE, just want more upper pole fullness after kids. 5'11", 40 YO, mother of 2 Breastfed both. Have recently lost weight and a lot of what Breast tissue I had left (especially in upper pole) Currently weigh 172lbs. PS says I measure a 15cm width and don't need a lift now, but will in future. He suggests Natrelle Moderate Plus implant for me. I trust his assessment, but based on my width measurement, I'd have to have an over 600CC implant. That thought terrifies me. I don't want HUGE boobs now, or the sagging that come with huge implants later. Help

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