Why is compressed mp4 twice the size when uncompressed

[TopskyIII]

Anyone downloaded from web rar files with mp4 content, which double it's MB size when unrared?
rar is 500 MB and unrared mp4 1050MB.
Normally video content is already compressed and when in rar shouldn't compress any more.
Any clue why and what is the best fix or work flow for a real lossless compress?

It seams that someone intentionally invents ways to enlarge download capacities.

GSpot text export:

FILE_NAME .................mp4
FILE_NAME_WITH_PATH D:\..\GSpot270a\...................mp4
FILE_SIZE 1,077,404,535
CONT_AUDIO_STREAM_COUNT 1
CONT_BASETYPE Undetermined MP4 (.MP4) file
CONT_BYTES_MISSING
CONT_INTERLEAVE_ALIGN
CONT_INTERLEAVE_PRELOAD
CONT_INTERLEAVE_TIME
CONT_SUBTYPE
CONT_TOTAL_BITRATE 0
VIDEO_ASPECT_CONVERT_AVI1
VIDEO_ASPECT_CONVERT_AVI2
VIDEO_ASPECT_CONVERT_CVD1
VIDEO_ASPECT_CONVERT_CVD2
VIDEO_ASPECT_CONVERT_DVD1
VIDEO_ASPECT_CONVERT_DVD2
VIDEO_ASPECT_CONVERT_SVCD1
VIDEO_ASPECT_CONVERT_SVCD2
VIDEO_ASPECT_CONVERT_VCD1
VIDEO_ASPECT_CONVERT_VCD2
VIDEO_ASPECT_SOURCE_MATCH
VIDEO_ASPECT_TYPE_NTSC
VIDEO_ASPECT_TYPE_PAL
VIDEO_BITRATE 5240
VIDEO_CODEC_NAME H.264/MPEG-4 AVC
VIDEO_CODEC_STATUS Codec Status Undetermined
VIDEO_CODEC_TYPE avc1
VIDEO_DAR 1.778
VIDEO_DURATION 26:45.405
VIDEO_FIELDS_PER_SEC
VIDEO_FRAME_COUNT 48114
VIDEO_FRAMES_PER_SEC 29.971
VIDEO_H264
VIDEO_MPEG2
VIDEO_MPEG2_3X2
VIDEO_MPEG2_BFF
VIDEO_MPEG2_I_L
VIDEO_MPEG2_PPF
VIDEO_MPEG2_PROG
VIDEO_MPEG2_TFF
VIDEO_MPEG4
VIDEO_MPEG4_BVOP
VIDEO_MPEG4_GMC
VIDEO_MPEG4_NVOP
VIDEO_MPEG4_QPEL
VIDEO_PAR
VIDEO_PICS_PER_SEC 29.971
VIDEO_QF 0.190
VIDEO_SAR 1.778
VIDEO_SIZE_X 1280
VIDEO_SIZE_Y 720
AUDIO_BITRATE 128
AUDIO_BITRATE_TYPE
AUDIO_CHANNEL_COUNT 2
AUDIO_CODEC mp4a: MPEG-4 AAC LC
AUDIO_CODEC_STATUS Codec Status Undetermined
AUDIO_MPEG_STREAM_ID
AUDIO_MPEG_SUBSTREAM_ID
AUDIO_SAMPLE_RATE 44100