3 weeks ago had an implant reduction from 450cc to 300cc. Could he have made them smaller? (photos)

150cc is a significant size decrease and should be visible.  If your skin is lax and a smaller implant is used, your breasts may appear more deflated and drooping.  A breast lift (skin tightening) " procedure might also be required if a smaller implant were used.  This of course would require additional scars.  I would talk with your Plastic Surgeon but his rationale appears reasonable.  

A small change in CC's may not be drastic enough to notice a significant size increase. Depending on the reason for the breast revision determines the sizing outcome. However, there are no guarantees with size predictions for revisions. Initially, if a woman wears a size 36 AB bra and would like to be a C cup, which is the average size that women wear and the average size bra that is sold, she needs a volume of approximately 350 to 390 cc to make that change. This becomes less if she's wearing a 34 bra, because it has a smaller strap size and the cups have a smaller diameter. A smaller diameter implant would then be a volume of approximately 300 to 350 cc. In the same way if she was more petite, maybe a double zero or size one and wore a 32 AB bra, then to become a 32C cup she would only require about 250 to 300 cc. This is a general number, and of course there are other factors that will affect this. 
Other factors which contribute to the outcome are #size, #shape, and #profile.  The #profile is determined by the amount of #projection versus roundness that the you desire, as well as the #size of the implant and size of your chest. Also, the placement or incision location will also have an effect on profile. Another element to a satisfying result of your #augmentation is realistic #expectations.#Shaped implants are all #textured and should not rotate when used properly. In my practice, about 20% of the patients benefit from shaped implants. Your Plastic Surgeon needs to measure you and determine the best shape and size of any implants that are used.

Although I understand your concerns, online consultants will not be a good position to "second-guess" your plastic surgeon who was there and had the benefit of assessing your tissues, making measurements... I think that you already have a good understanding of the issues at hand; at this point, time and patience will be your best friends as the breast skin and tissues "bounce back". It is possible, that you will be happier down the line. On the other hand,  if you remain displeased with your breast size, further downsizing (which may also involve capsulorraphy and/or breast lifting), may be "necessary" in the future. You may find the attached link, dedicated to breast implant downsizing surgery concerns, interesting to you. Best wishes.

In my experience, usually takes at least hundred cc of either increase or decrease to make it obvious that a change has been made. At 150 mL, there should be an appreciable, probably 1/2 cup size reduction in breast volume. If you're trying to avoid some removal of skin such as in a breast lift, then there is a balance between how small the implant can be, without the necessity of tightening the skin around it, such that the implant is merely hanging and not filling the top of the breast. Best of luck with your recovery.