Snapchat only allows videos of up to 2.5MB to be shared. Therefore, if your camera roll video is bigger than 2.5MB, you need to trim or compress the video size. This video show the best way to compress video for using in Snapchat.
Official Guide(More details here): http://www.multipelife.com/compress-video-for-snapchat.html
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Pavtube iMedia Converter for Mac: http://www.multipelife.com/imedia-converter-mac
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