It seems like generosity: give a plant a big pot now, and it won't need repotting again for years. In practice, it's one of the more common ways people accidentally cause root rot. A pot that's much larger than the root ball holds a large volume of soil the roots aren't using yet, and that unused soil stays wet far longer than the plant can drink it down.
The sizing rule
When repotting because a plant has outgrown its container, go up by about 1–2 inches in diameter, not dramatically larger. Fast-growing plants can sometimes handle a slightly bigger jump, 2–4 inches, but for most houseplants and most repottings, the smaller step is the safer one. This holds whether you're measuring in inches or centimeters. The underlying idea is the same: match the pot to the current root ball, not to how big you'd eventually like the plant to get.
Drainage matters more than size
A pot needs a way for excess water to leave, regardless of how well the size is judged. Pots without drainage holes hold water at the bottom no matter how carefully you measure a watering, the classic workaround is to keep the plant in a plain nursery pot with holes, and set that pot inside a decorative one as a cachepot, lifting it out to drain fully after watering rather than letting water collect at the bottom of the decorative pot.
Material affects how forgiving the pot is
- Plastic: lightweight, holds moisture longer, and is a reasonable default for anyone prone to underwatering or for hanging planters where weight matters.
- Terracotta (unglazed clay): porous, dries out faster, and is well suited to plants that prefer their soil to dry out fully between waterings, succulents, cacti, and generally anyone who tends to overwater, since the material itself helps compensate.
- Glazed ceramic: behaves more like plastic in terms of moisture retention, with the visual weight of terracotta; still needs a drainage hole to function well.
None of these fully substitutes for correct sizing, a plastic pot that's also oversized will still hold excess moisture, just more so than a correctly-sized one would.
Matching pot depth to root shape
Beyond diameter, depth matters for some plants more than others. Shallow-rooted plants (many succulents, for instance) don't benefit from a tall, deep pot; the extra soil below the roots just stays wet unused. Matching the pot's shape to how the plant actually roots, not just picking whatever looks good, avoids creating that same excess-moisture problem in a different dimension.