i've noticed that the buffers that dvdsubdec generates were 6635520 bytes each. 

/GstPipeline:pipeline0/GstDvdSubDec:dvdsubdec0.GstPad:src: caps = video/x-raw, format=(string)ARGB, width=(int)720, height=(int)576, framerate=(fraction)0/1

that's 4 times the expected size of a PAL sized ARGB bitmap of 720*576*4 = 1658880

i suspect a bug which was introduced during porting to new api
the original allocation line was:
gst_pad_alloc_buffer_and_set_caps (dec->srcpad, 0, 4 * dec->in_width * dec->in_height, GST_PAD_CAPS (dec->srcpad), &out_buf);
while the new one uses:
out_buf = gst_buffer_new_allocate (NULL, 4 * GST_VIDEO_INFO_SIZE (&dec->info), &params);

with a macro which i don't know yet but it's obviously already considering the bit depth per pixel, so that the extra 4 * results in a buffer that's 128bpp instead of 32bpp.

propsed fix to be attached.