4.6gb for a single layer smart object 18" by 12" inch. Why?

[Hmmokthen]

Hello,

I had a file at 2.4gb with multiple layers including smart objects. Im trying to re-size the image into 18" by 12", which means making the image slightly thinner and taller. Once re-sized I merged all the layers into a single smart object and then applied a smart sharpen (to counter any softness created in the re-sizing).

File size has gone from 2.4 to 4.6gb! and therefore I cannot save as a tif. Makes no sense to me. Anyone with any thoughts on thisUPDATE: I experimented with a copy of the file by rasterising the single layer — goes down from 4.6gb to 200mb!

UPDATE 2: So I started again, this time I flattened the image and converted to smart object BEFORE resizing (done via "automatic" resample as I dont know which one to select if one end is enlarging and the other is downsizing) and wouldn't you know, its only 400 odd mb. 

[Bojan Živković · Accepted solution]

When you convert multiple layers into a Smart Object layer, Photoshop retains all layers and the data within them in the background, making it accessible whenever needed. It's not surprising that file sizes have increased, even though the number of layers has decreased.

[Hmmokthen]

thx, will avoid doing that next time

[c.pfaffenbichler]

Smart Objects are tricky with regard to file size … 

A document containing nothing but the unscaled flat Smart Object can have roughly 3x the size of the SO – the SO, the SO in Layers, the complete file’s preview. (Though it also depends on compression and file saving settings.)

If the SO however is not flat, even if it only contains that one Layer, the containing file can have roughly 4x the SO’s size, because the SO now contains the image and a preview.