There are many factors that go into sizing of implants-your desires, tissues and your measurements. The measurements are a guide and put the surgeon and the patient "in the ballpark" for what fits for your body. Also the measurements aren't always exact and reproducible amongst different surgeons doing the measuring.
The Allergan Style 20 High Profile 300 cc device has a base dimension of 10.9 cm. If your true breast base width is 10.0 cm then you can expect the 300 cc HP(10.9cm wide) to extend beyond the natural boundaries of your breast. This doesn't necessarily mean that it is "wrong", but it may come with some "trade-offs".
When implants are placed that are much too wide for the breast then you may feel or see the edge of the implant more easily. When high profile implants are placed that are much too wide there may be, depending on your tissues, and increased stretch which can contribute to the implants ending up too close together or disrupting the fold below the breast and the device ending up too low.
The quality and elasticity of your tissues is important if you are going to consider and implant that is wider than your true breast dimension. It may be worth seeing your surgeon again, getting re-measured, and if your base width is 10cm, discussing what the particular trade-offs for you may be.