In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

When an I²C line (SDA or SCL) is pulled low, this is done by a device output with relatively low impedance, so the potential on the line changes relatively fast from high to low. On the other hand, when the line shall go high, the device output goes to high impedance to release the line, and the line is then brought to high level by a current that flows through the pull-up resistor. So, a smaller resistor means a faster rise-time. Nevetheless, the resistor must be large enough to limit the current flowing when a device pulls the line low to a value below the maximum allowed device output current. In the Microchip 24C04 datasheet, I did not find a spec. for the maximum allowed output current but a current of 3mA is specified together with the low level output voltage, so pull-up resistors of 2.2k or higher @ 5V supply limit the current to values below 3 mA.

On longer bus lines, lower pull-up resistors, or several resistors in parallel distributed along the bus and at both ends help to also reduce any unwanted reflections.

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