Important lightmap setting

if Lightmap size is more bigger than previous it, you should check following options.

Lightmap settings

Property:	Function:
Scale in Lightmap	This value specifies the relative size of the object’s UVs within a lightmap. A value of 0 will result in the object not being lightmapped, but still contribute to lighting other objects in the scene. A value greater than 1.0 increases the number of pixels (ie, the lightmap resolution) used for this object while a value less than 1.0 decreases it. You can use this property to optimise lightmaps so that important and detailed areas are more accurately lit. For example: an isolated building with flat, dark walls will use a low lightmap scale (less than 1.0) while a collection of colourful motorcycles displayed close together warrant a high scale value.
Prioritize illumination	Check this box to tell Unity to always include this object in lighting calculations. Useful for objects that are strongly emissive to make sure that other objects will be illuminated by this object.
Lightmap Parameters	Allows you to choose or create a set of Lightmap Parameters for the this object.

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