what is wrong with this ffmpeg command? -

i trying convert 3gp movie h264 video (mp4) using:

ffmpeg -i file8.3gp -vcodec libx264 -preset slow -vf scale="720:trunc(ow/a/2)*2" -threads 0 -acodec libvo_aacenc -b:a 128k "file8.mp4" 

the problem original file 28kb after conversion 155kb mp4 file why file bloated - because of size or other option?

here see on 3gp input file: note bit rate 46 kb/s

libavutil      52. 39.100 / 52. 39.100   libavcodec     55. 18.102 / 55. 18.102   libavformat    55. 12.102 / 55. 12.102   libavdevice    55.  3.100 / 55.  3.100   libavfilter     3. 80.101 /  3. 80.101   libswscale      2.  3.100 /  2.  3.100   libswresample   0. 17.102 /  0. 17.102   libpostproc    52.  3.100 / 52.  3.100 input #0, mov,mp4,m4a,3gp,3g2,mj2, 'file8.3gp':   metadata:     major_brand     : 3gp5     minor_version   : 256     compatible_brands: 3gp53gp4     creation_time   : 2005-10-28 17:36:40   duration: 00:00:04.93, start: 0.000000, bitrate: 46 kb/s     stream #0:0(eng): audio: amr_nb (samr / 0x726d6173), 8000 hz, mono, flt, 8 k b/s     metadata:       creation_time   : 2005-10-28 17:36:40       handler_name    : apple sound media handler     stream #0:1(eng): video: mpeg4 (simple profile) (mp4v / 0x7634706d), yuv420p , 176x144 [sar 1:1 dar 11:9], 35 kb/s, 15 fps, 15 tbr, 600 tbn, 1k tbc     metadata:       creation_time   : 2005-10-28 17:36:40       handler_name    : apple video media handler 

here see on mp4 output file: note bit rate 243 kb/s

libavutil      52. 39.100 / 52. 39.100   libavcodec     55. 18.102 / 55. 18.102   libavformat    55. 12.102 / 55. 12.102   libavdevice    55.  3.100 / 55.  3.100   libavfilter     3. 80.101 /  3. 80.101   libswscale      2.  3.100 /  2.  3.100   libswresample   0. 17.102 /  0. 17.102   libpostproc    52.  3.100 / 52.  3.100 input #0, mov,mp4,m4a,3gp,3g2,mj2, 'file8.mp4':   metadata:     major_brand     : isom     minor_version   : 512     compatible_brands: isomiso2avc1mp41     encoder         : lavf55.12.102   duration: 00:00:05.20, start: 0.200000, bitrate: 243 kb/s     stream #0:0(eng): video: h264 (high) (avc1 / 0x31637661), yuv420p, 720x588 [ sar 539:540 dar 11:9], 240 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc     metadata:       handler_name    : videohandler     stream #0:1(eng): audio: aac (mp4a / 0x6134706d), 8000 hz, mono, fltp, 10 kb /s     metadata:       handler_name    : soundhandler @ least 1 output file must specified 

try -vf "scale=720:trunc(ow/a/2)*2"

have nice day ;)