Only devices with Android 4.1+ are able to do video compression in Telegram. It is related with the resolution of the video that is sent, too. It has to be bigger than 640px in width or height. It's disabled for some devices too because some Androids (4.1 and 4.2) don't have a public API for video encoding.
Happens on Android 4.3 and older devices. It's an Android problem, which means that the device vendor didn't pass CTS and released the device without checking. On 4.3+ it can't be fixed by the Telegram Android developer anymore because on these Android versions Telegram is using only the Android API for conversion.