Resolve the difference in PDF size between two variants of document produced by DOC1 Generate

Two very similar PDF files (with basic text layouts) are very different sizes despite not embedding the fonts.

One document uses Trebuchet TrueType, the other uses ArialMT.

The extra space is due to the different font. While the font is not embedded as such, the font you have chosen for the larger document has a CIDFont as a descendent. This causes sections like this to be embedded in the PDF:
--------------------------------------------------
/Type /Font
/Subtype /Type0
/Name /G1F1
/BaseFont /ArialMT
/DescendantFonts[ <<
/Type /Font
/Subtype /CIDFontType2
/BaseFont /ArialMT
/WinCharSet 254
/FontDescriptor 8 0 R
/CIDSystemInfo <<
/Registry (Adobe)
/Ordering (Identity)
/Supplement 0 >>
--------------------------------------------------
...followed by the glyph information PDF requires for such fonts.

A brief description of how PDF handles these fonts can be found here: http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/API_References/Acrobat_API_Reference/PD_Layer/PDFont.html :
--------------------------------------------------
Type 0 fonts may have a CIDFont as a descendent. A CIDFont is designed to contain a large number of glyph procedures and is used for languages such as Chinese, Japanese, or Korean. Instead of being accessed by a name, each glyph procedure is accessed by an integer known as a character identifier or CID. Instead of a font encoding, CIDFonts use a CMap to define the mapping from character codes to a font number and a character selector. For more information on CIDFonts, see the following sections in the PDF Reference:

Section 5.6.1, CID-Keyed Fonts Overview
Section 5.6.2, CIDSystemInfo Dictionaries
Section 5.6.3, CIDFonts
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The full reference can be downloaded here: http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf