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This article explains why diagonal lines look wider than vertical or horizontal lines when displayed on the screen or when printed to raster or PostScript printers, although the lines are given the same point thickness.
DISCUSSION
QuickDraw is the part of the Macintosh Toolbox that performs graphics operations on your screen. QuickDraw provides applications developers with routines for rapidly creating, manipulating, and drawing graphic objects, such as lines, arcs, rectangles, ovals, regions, bitmaps, and so on.
QuickDraw provides a graphics pen to perform some drawing operations. This graphics pen is rectangular in shape, and its size, height, and width is measured in pixels. The default size is 1 x 1 pixel square. Therefore, when you draw a 45 degree diagonal line with a square pen (for example, penSize.h == penSize.v), the apparent thickness is roughly 1.4 times the penSize, less thick at lesser angles to the horizontal or vertical axis. See Figure 1.
Figure 1 Horizontal and diagonal lines
If your graphics program does not adjust for off-axis drawing of lines, you may be able to approximate the difference in size of the line width and adjust the pen manually.
Additional information may be found in Inside Macintosh Imaging with QuickDraw, Chapter 3 on QuickDraw Drawing.