I noticed that when I export photos from LR to my hard drive size of the files are much smaller than before. I save all my photos in high 600ppx resolution and 2176x3264pixels size. Before saved files had about 3-5MB now with the same px size is only about 700-1000KB. Will it affect the quality of the photos?
As JPEG? If so, nothing to worry about and to be expected.
Can you please explain what you mean by "px size"? If you saved your photos before at 2176x3264 pixels, do they now have the same number of pixels, or fewer pixels?
The "resolution" figure (in pixels-per-inch) may not be relevant at all. Other factors affecting file size are degree of lossy compression (assuming JPG, this is labelled as rhe Quality setting) and the degree to which the photo as shot and then as processed, shows lots of fine detail contrasts from one pixel to the next. Those fine detail contrasts might be genuine picture detail, especially of a fully textured rather than very smooth surfaced subject, or equally, they may be image noise of one sort or another.
Sharpening or added Grain can therefore increase export filesize - by making a photo harder to represent with a small file size JPG, and conversely noise reduction can get rid of spurious non-detail, and so make it easier to represent with a small file size. But the biggest factor is generally the number of pixels in the exported file - regardless of how many / few "Pixels per Inch" those may be set to.
There are a few factors that will impact the file MB size beyond the total number of pixels. Including metadata with the exported file can increase the file size. Not including metadata will likely reduce the size. Exporting to tiff will create a larger file than a jpg even if they have the same number of pixels. Exporting a jpeg with a lower quality number will reduce the size too.
As mentioned above, the ppi number has no significance for the quality of the digital image itself or the file size. It is only an instruction for a printer regarding the size of the print. In your case 2176x3264 at 600 ppi means a 3.6 x 5.4 inch print, around post-card size. Should you want a larger print, the printer will simply disregard the embedded ppi number, in effect using a smaller ppi (as print size goes up, ppi goes down and vice-versa) until the ppi gets too small for acceptable quality. 300 ppi is usually considered ideal and 150 ppi roughly the bottom limit. I say roughly because other factors like viewing distance also come into play.