You cannot always rely on the results from the first page to be the same for all the rest. I've seen enough mixed format PDFs out there in the wild to not want to base any code on that assumption.
A more reliable way to determine the media sizes for each page (and even each one of the embedded {Trim,Media,Crop,Bleed}Boxes) is the commandline tool pdfinfo.exe (part of the XPDF tools from http://www.foolabs.com/xpdf/download.html). You can run the tool with the "-box" parameter and tell it with "-f 3" to start at page 3 and with "-l 8" to stop processing at page 8.
Example output:
C:\downloads>pdfinfo -box -f 1 -l 3 _IXUS_850IS_ADVCUG_EN.pdf
Creator: FrameMaker 6.0
Producer: Acrobat Distiller 5.0.5 (Windows)
CreationDate: 08/17/06 16:43:06
ModDate: 08/22/06 12:20:24
Tagged: no
Pages: 146
Encrypted: no
Page 1 size: 419.535 x 297.644 pts
Page 2 size: 297.646 x 419.524 pts
Page 3 size: 297.646 x 419.524 pts
Page 1 MediaBox: 0.00 0.00 595.00 842.00
Page 1 CropBox: 87.25 430.36 506.79 728.00
Page 1 BleedBox: 87.25 430.36 506.79 728.00
Page 1 TrimBox: 87.25 430.36 506.79 728.00
Page 1 ArtBox: 87.25 430.36 506.79 728.00
Page 2 MediaBox: 0.00 0.00 595.00 842.00
Page 2 CropBox: 148.17 210.76 445.81 630.28
Page 2 BleedBox: 148.17 210.76 445.81 630.28
Page 2 TrimBox: 148.17 210.76 445.81 630.28
Page 2 ArtBox: 148.17 210.76 445.81 630.28
Page 3 MediaBox: 0.00 0.00 595.00 842.00
Page 3 CropBox: 148.17 210.76 445.81 630.28
Page 3 BleedBox: 148.17 210.76 445.81 630.28
Page 3 TrimBox: 148.17 210.76 445.81 630.28
Page 3 ArtBox: 148.17 210.76 445.81 630.28
File size: 6888764 bytes
Optimized: yes
PDF version: 1.4