Ubuntu 16.4.1 iso size 1Gb on ubuntu site. but when I download via torrent it seems iso size 1.5 GB. which one should I download?
Torrent: http://releases.ubuntu.com/16.04.1/ubuntu-16.04.1-desktop-amd64.iso.torrent
Ubuntu site: http://www.ubuntu.com/download/desktop/contribute?version=16.04.1&architecture=amd64
You are probably referring to the formal size limit for Ubuntu releases. On paper, the release ISOs may not be larger than 1 GB. But in reality, the last few releases did not observe that limit and were larger. The ISO image for the desktop edition of Ubuntu 16.04 is around 1.4 GB to 1.5 GB, so your number sounds about right. The ubuntu-16.04.1-desktop-amd64.iso.torrent should be the correct one. There already is a discussion to raise the formal size limit. For further information, see for example this article.
Have a look on the article linked below the download that tells you how to verify the isos: http://www.ubuntu.com/download/how-to-verify If one of those images was tampered with you can prove it this way.
