1x, 2x and 4x need to be that ratio in size. The hint is in the names...
If you want to add more platforms - its officially unsuported, but you can do so in tk2dSystem.cs,
public tk2dAssetPlatform[] assetPlatforms = new tk2dAssetPlatform[] {
new tk2dAssetPlatform("1x", 1.0f),
new tk2dAssetPlatform("2x", 2.0f),
new tk2dAssetPlatform("4x", 4.0f),
};
Add another 2x_NoCompression or something like that. The 2.0f refers to the relative scale, so 2x_NoCompression should be 2.0f.