Más grande vs. mayor
While both más grande and mayor can translate to bigger, larger, or greater, their differentiation depends on whether you are describing physical dimensions, age, or abstract intensity.
Más grande
A1This phrase is used primarily to describe physical size, dimensions, volume, or scale of tangible objects. If you can measure the object with a ruler or scale, más grande is usually the correct choice.
Tu casa es más grande que la mía.
(Your house is bigger than mine.)
Necesito una talla más grande de estos zapatos.
(I need a bigger size of these shoes.)
Este coche es más grande, pero consume más gasolina.
(This car is larger, but it consumes more gas.)
Júpiter es el planeta más grande del sistema solar.
(Jupiter is the largest planet in the solar system.)
Quiero la porción de pastel más grande.
(I want the biggest slice of cake.)
Mayor
A1This word is the irregular comparative form of grande. It is most commonly used to mean older (referring to age), greater (referring to abstract intensity or quantity), or major (referring to importance). In mathematics, it signifies greater than.
Mi hermano es mayor que yo.
(My brother is older than me.)
El riesgo de infección es mayor si no te lavas las manos.
(The risk of infection is greater if you do not wash your hands.)
Siete es mayor que cinco.
(Seven is greater than five.)
Su mayor logro fue terminar la carrera.
(His greatest achievement was finishing the race.)
Necesitamos una mayor eficiencia en la empresa.
(We need greater efficiency in the company.)
Las personas mayores tienen descuento.
(Older people have a discount.)
Summary
Use más grande when referring to the physical size of objects or spaces. Use mayor when referring to a person's age (older), mathematical values (greater than), or the intensity and importance of abstract concepts.