The DSLR-A900 starts with a 24 x 35.9mm size CMOS sensor developed especially for this camera.  Though that imaging canvas is utterly packed with pixels, the pixels themselves dominate the landscape of the image receiving area, instead of the elements of image sensors which are necessary but not actually collecting the image.  Indeed, the A900's pixels are larger than the fine consumer-grade Alpha 700, despite having twice as many pixels.

On the physical side of things, the Alpha's designed for professional use:  all operations are weather-sealed, its high-performance shutter's tested to 100,000 cycles, and its viewfinder (the forgotten critical SLR element in the age of squinty APS-C size viewfinder dominance) is as good as they come.  Despite these claims, together with batteries and a memory card it tips the scales at ~2 lbs (900g), a good competitive number especially considering the enjoyable view.