This is completely normal. A demoisaiced D7100 file has 24.1 MP in 3 channels (R,G,B). The default is 16 bits sent to Photoshop, which means a single layer takes 3*(16/8)*24.1=145MB. Three layers of this and you are at your 450 MB. Rotate and move these a bit and you'll get to 520 MB very quickly. When you save, because a preview layer has to be added (compatibility mode), this will go up to 600MB, which with compression will go down to 300-400MB. Nothing you can realistically do about this except for flattening the photoshop image before saving. This will make your image go down to 145 MB again, which with lossless compression will go to about 70-80MB. If you want to keep your layers, you are stuck with the large files. You could conceivably use only 8-bits images (also go down to adobeRGB color space if you do that to avoid posterization that can occur if you stay in prophotoRGB but go to 8 bits) which gains you a factor two in file size.
In contrast, a raw file has only a single channel per pixel (the Bayer mosaic that most cameras use has only one color channel per pixel!) usually in 12 or 14 bits resolution, which means that an uncompressed 24.1 MP 12-bits raw file is 24.1*12/8= 37 MB. With lossless compression this will go down to 26MB or so. However, you cannot edit raw images in Photoshop! Photoshop requires demosaiced images with multiple channels per pixel.
Thanks for your response. File management has always been a moderate point of confusion for me, but this sheds some much needed light. Is there any export option to help keep multiple layers from raw files from becoming too cumbersome before they go into Photoshop, converting to different formats, for example?