texture memory problem

texture memory problem
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i have an png file (4006x2048)
i load it into game using

CCTexture2D::setDefaultAlphaPixelFormat(kTexture2DPixelFormat_RGBA4444); mTexture = CCTextureCache::sharedTextureCache()->addImage(texturePath);

so i calculate 4006*2048*4 = 32,817,152 ~ 30Mb
so i think it’ll cost ~30Mb of ram
but not, it’s cost about 50Mb, memory grow from 17Mb to 66Mb
i don’t understand.