oh a pic have 3 views type , i know now
but  pics  in 'pwg_high' folder(the same as the source pics) smaller than the out of one. why ?

A simple way is making a template-extension to replace the default picture_content.tpl
Please, make a search to find how to use template-extensions or ask in another topic if you don't know.

Instead of coding:

<img src="{$SRC_IMG}" style="width:{$WIDTH_IMG}px;height:{$HEIGHT_IMG}px;" alt="{$ALT_IMG}"

You can replace width:{$WIDTH_IMG}px by something like:

width:{if is_a_guest()}{math equation="floor(x/3)" x=$WIDTH_IMG}{else}{$WIDTH_IMG}{/if}px

(divide the size by 3 or more, and follow this solution for the height too).
I didn't try it but I am sure you could be satisfied with it.
Enjoy!

Note: avoid adding space or newline in the above code (may have a direct impact on the result).

Source: http://smarty.net/manual/en/language.function.math.php
(Please read technical notes)
is_a_guest() is a Piwigo-function always available at this stage.