Will a mattress grow in size over time after delivery?

Mattresses do not generally get bigger over time after delivery.
The size of a mattress is determined during the manufacturing process and will not change significantly under normal use. However, there are a few factors that can cause a mattress to expand slightly:
New mattress odor: New mattresses may release some odors, which can cause the mattress to expand slightly. These odors will usually dissipate within a few days or weeks, and the mattress will return to its normal size.
Improper use: If a mattress is used improperly, such as by frequently sitting on the edge of the mattress or jumping on the bed, it may cause the mattress to deform or expand.