Did some further digging with tcpdump:

tcpdump -i pppoe0 dst port ftp

Resulted in the following, the first being the excerpt after doing a get
of a 280 byte file successfully, the second being the attempt at a
667559 byte file unsuccessfully.

13:38:30.840142 PPPoE  [ses 0x78b] IP 50: x.x.x.x.65492 > z.z.z.z.ftp: P
1501725748:1501725756(8) ack 1170470097 win 33580 (DF) [tos 0x10]
13:38:31.385934 PPPoE  [ses 0x78b] IP 68: x.x.x.x.65492 > z.z.z.z.ftp: P
8:34(26) ack 20 win 33580 (DF) [tos 0x10]
13:38:31.892593 PPPoE  [ses 0x78b] IP 48: x.x.x.x.65492 > z.z.z.z.ftp: P
34:40(6) ack 29 win 33580 (DF) [tos 0x10]
13:38:32.606017 PPPoE  [ses 0x78b] IP 42: x.x.x.x.65492 > z.z.z.z.ftp: .
ack 77 win 33580 (DF) [tos 0x10]
13:38:32.974743 PPPoE  [ses 0x78b] IP 68: x.x.x.x.65492 > z.z.z.z.ftp: P
40:66(26) ack 77 win 33580 (DF) [tos 0x10]
13:38:33.756341 PPPoE  [ses 0x78b] IP 42: x.x.x.x.65492 > z.z.z.z.ftp: .
ack 154 win 33580 (DF) [tos 0x10]
13:38:34.167078 PPPoE  [ses 0x78b] IP 68: x.x.x.x.65492 > z.z.z.z.ftp: P
66:92(26) ack 178 win 33580 (DF) [tos 0x10]
13:38:34.886325 PPPoE  [ses 0x78b] IP 42: x.x.x.x.65492 > z.z.z.z.ftp: .
ack 198 win 33580 (DF) [tos 0x10]

And the second:

13:38:55.833130 PPPoE  [ses 0x78b] IP 64: x.x.x.x.65492 > z.z.z.z.ftp: P
1501725840:1501725862(22) ack 1170470294 win 33580 (DF) [tos 0x10]
13:38:56.513821 PPPoE  [ses 0x78b] IP 48: x.x.x.x.65492 > z.z.z.z.ftp: P
22:28(6) ack 13 win 33580 (DF) [tos 0x10]
13:38:57.416020 PPPoE  [ses 0x78b] IP 42: x.x.x.x.65492 > z.z.z.z.ftp: .
ack 61 win 33580 (DF) [tos 0x10]
13:38:57.814823 PPPoE  [ses 0x78b] IP 64: x.x.x.x.65492 > z.z.z.z.ftp: P
28:50(22) ack 61 win 33580 (DF) [tos 0x10]
13:38:58.586024 PPPoE  [ses 0x78b] IP 42: x.x.x.x.65492 > z.z.z.z.ftp: .
ack 137 win 33580 (DF) [tos 0x10]

The following is on screen after doing the get:
local: pcre-5.0.tar.gz remote: pcre-5.0.tar.gz
229 Entering Extended Passive Mode (|||46796|)
150 Opening BINARY mode data connection for pcre-5.0.tar.gz (667559
bytes)

And no further activity occurs either in tcpdump or in ftp client.

Relevent interfaces:

pppoe0: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1492
        inet x.x.x.x -> y.y.y.y netmask 0xff000000
        inet6 fe80::240:95ff:fe04:5e70%pppoe0 ->  prefixlen 64 scopeid
        0xb

which uses:

ne2: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu
1500
        address: 00:40:95:04:5e:70
        media: Ethernet manual
        inet 192.168.1.2 netmask 0xffffff00 broadcast 192.168.1.255
        inet6 fe80::240:95ff:fe04:5e70%ne2 prefixlen 64 scopeid 0x1

and from dmesg:

ne2 at pci0 dev 8 function 0: Winbond 89C940F Ethernet
ne2: Ethernet address 00:40:95:04:5e:70
ne2: interrupting at irq 11