I used to use Mac's Photos application, but I don't like the fact that it's generating some huge Library file which might get corrupted and without mac I can't access my images. And if I want to have pictures stored as files, I need to export them, so my data are actually twice as huge. But I really like the easy to use button 'import all new photos' so everything new is imported automatically. Do you know any alternative with such functionality, but storing all pictures as files?
I used to use Mac's Photos application, but I don't like the fact that it's generating some huge Library file which might get corrupted and without mac I can't access my images.
Frankly, I think you're creating a non-issue.
The Library "file" is not a single file, but an OSX package. It's simply a directory that displays as if it were a single file in the Finder. You can easily view the contents of the package by right-clicking (control-clicking) on the library "file", and selecting Show Package Contents. If you do this, you'll see the old familiar pre-package iPhoto Library contents, including your Masters folder with the individual image files.
If you are conversant with UNIX commands, the library file appears as a simple directory in the terminal, and you can do all the usual UNIX commands to copy/export your files from the Library to wherever you want. (Of course, you might also completely screw up the file structure and organization that Photos relies on, which is why Apple made this directory a package.) And if you copy the Library "file" to another OS, it will appear as the directory it is, because you aren't using the OSX Finder to display it.
my data are actually twice as huge
JPEG file size is ~10MB and RAW ~25, which makes ~35MB per picture, but if you have ~100 GB of raw+jpeg files, then total amount is 200 GB after exporting (I know without jpeg it could be ~70 GB, but jpegs might be handy some times)
A solution could be to simply stop shooting both RAW and jpeg all the time and only do it for those occasions it actually is handy.
Most camera manufacturers provide software to convert RAW files to jpeg files the way the camera would do it (as opposed to using some third party RAW converter). This way you can always get the jpeg from the RAW if you need it in post.
This limits the necessity to shoot RAW + jpeg to those occasions when you need jpeg on the camera (say for example when you need to connect the camera directly to a printer).
And if I want to have pictures stored as files, I need to export them, so my data are actually twice as huge.
If you want to have pictures "stored as files" then what's the point of using photo management software in the first place? As inkista pointed out, your photos are files in the Photos library, so there's really no point in exporting them just to have more files that you have to manage manually. Your best bet is to let Photos manage the images, but backup your machine regularly. (Time Machine, built into MacOS X, is so easy to use and effective that it's crazy not to use it.)
Do you know any alternative with such functionality, but storing all pictures as files?
If you're really jonesing to see all the image files in your file system, switch to Adobe Lightroom. Lightroom doesn't use a central library, but instead lets you put the files where you want. That sounds like what you're asking for, but you need to realize that you have to be careful not to move the files because Lightroom won't know where to find them if you do.