Yes & yes, goto your Instant uploaded photos or shared photos on your G+ Profile (or, mine, since I have plenty of public photos), then, 'download full size' under 'options'. You'll get 2048x1536 picture. The originals should be larger if using today's digital camera or modern smartphones. My mobile phone takes 8MP photos, so they should be 3264x2448 pixs. FB also does the same compressing of photos (if larger than 2048x1636).
So, if you want to preserve the original dimension/size, then, use a file sharing/online storage service, i.e. Google Drive.