this is normal. As you can see, you ticked "Bleed Marks". So INDD adds hairline rules that define the amount of extra area to image outside the defined page size. This obviously increases the output size.
If you want to export full size, untick any printer's mark and set Bleed and Slug Values to zero.
Besides, you may want to ask yourself why you need Bleed marks. They're usually not so useful. If you want to prepare file for pro printing, you need Crop marks AND bleed set to 3.
Thanks for answering so quickly!
The document is going to be an A4 booklet to be printed by a professional printer so I have added the 3mm all around to be trimmed off by the printer (I have some images that I want to print right to the edge of the page). However, the document is still to large even when the printer trims off the 3mm all around? With the 3mm bleed all around it should only add 6mm to both the width and height. In which case, shouldn't the dimensions be w:216mm x h:303mm? I'm still at a loss as to what Adobe PDR export preference I have messed up?
And when using bleed 3 mm, set Marks Offset to 3 mm too (your second screenshot, to place any print marks outside of bleed area). Also better turn off Bleed Marks and turn on Crop Marks, Registration Marks and Page Information).
As I said before, uncheck Bleed Marks (useless), check at least Crop marks AND set bleed to 3mm.
Registration Marks and Page Information can be turned on too, optional but your printer might find them helpful.
I advice using PDF/X-4 profile and, of course, export as Pages, not Spreads.
You didn't mess anything up, your printer will trim at crop marks, regardless of PDF size.
Okay thanks again. I will do as instructed at work tomorrow and report back. I really appreciate your help so far.
Thanks rudo123. I'll try all this as soon as I am back at work. Fingers crossed!