Selecting the proper implant
The choice of implant often depends on the patient's anatomy.
Generally speaking, higher profile implants have a narrower diameter and would work better in a patient with a smaller frame. Furthermore, high profile implant project forward more, so they will appear bigger than a low profile implant of the same volume.
Another factor to consider is that high profile implants tend to cause less rippling. This is especially important in thin individuals.
Based on your description, you have a small frame and may benefit form high profile implants.
Keep in mind that a a 425cc high profile will seem a lot bigger than a 420cc low profile implant because the high profile implant sticks out more.
Depending on your anatomy and the type of clothes you wear, the high profile implants should give you a significant noticeable difference through clothes.
Good luck with your surgery.
You probably will like the high profile
If you need a narrower implant to fit your chest width better, but want more projection than you have, a moderate plus or high profile implant will help you get the result you want. The high profile variety gives the most forward projection and fullness per cm of width.
A slightly larger high profile implant may be a good option
In my practice, I rarely use low profile implants for cosmetic augmentations, precisely because of the problem you're describing. A high profile implant of the same volume or slightly larger will probably give you the look you're going for, but it's hard to know without examining you. I would recommend seeking out a Board-Certified Plastic Surgeon in your area that you can consult with-- he wil be able to examine you and give you options for achieving the look you're going for.
If you feel the implants are too flat and too wide, then it makes sense that a narrow and higher projecting implant would be helpful. What specific size and style would depend on your breast width measurement, the location of the nipple relative to your breast and chest, and your desired post-op appearance. You might be able to judge your volume requirements depending if you are happy with the volume but not the shape.Also if you are wearing a padded bra that gives you the look you want, your surgeon can estimate the dimensions and volume and give you some idea of what implant will most closely match you goal. However, also remember that breast augmentation is an imperfect operation and your surgeon shold review what side effects each choice you make will entail (i.e. roundness, rippling, firmness, asymmetry, etc.).
High projection implants are a great way to achieve more projection with a narrower implant on a small frame.
Maybe you should switch to high profile implants
If you are looking for more projection to the extent that you feel the desire to wear a padded bra with current lower profile implants, then a higher profile may be for you. Have you discussed this with your surgeon? Are you sure you don't have "moderate profile?" That is what most of us place the first time around. This is all about dimensions. Find out what you currently have and go from there.