If raceway is too small, you can damage conductors while trying to pull them through. Even if they aren’t damaged during the pull, they may overheat inside an undersized raceway. For these reasons, the NEC has requirements for minimum raceway size.
You can size a raceway with this three-step method:
Step 1: Determine the cross-sectional area (in square inches) for each conductor from Chapter 9, Table 5 for insulated conductors and from Chapter 9, Table 8 for bar