Will 400cc hp silicone unders be too big for me? (Photo)

That is a very personal decision for you to make. Rule of thumb is always go larger than you want... 

At
size 34 B each 100 cc’s of implant corresponds to 1 cup size change.400 cc implants would take you up 4 cup sizes
to a F.400 cc implants will not fit
retro-pectoral, will extrude inferiorly and laterally requiring revision.I always recommend small round textured
silicone gel implants placed retro-pectoral since they look and feel more
natural, are more stable, less likely to ripple or have complications needing
revision.Implant profile is irrelevant
in the retro-pectoral position since the muscle compresses it.22% of women who undergo breast augmentation
require revision within 2 years because they are dissatisfied with the
result.A frequent cause of
dissatisfaction is over implantation.Best Wishes,Gary Horndeski, M.D.

Based only on your photographs a 400 cc high-profile implant could look nice on you and will certainly give you breasts close to the D cup range in most bras.It is important to revisit your plastic surgeon, ask the surgeon to take measurements of your chest and breast .   This will enable your plastic surgeon to demonstrate to you a chart which will list the various implant shapes and sizes that anatomically will fit your breast the best after you choose a breast profile that you want.  The difference between 375 and 400 cc breast implants is only about 2 tablespoons and should not be that different.